There is a wonderful old publishing house in Russia named MIR Publishers. I'm not sure if they still exist or not, but at one point in time, they released many interesting books about a wide range of subjects. A saint by the name of Damitr has put a large number of these on Internet Archive. As far as I know, they are legal to download.

Here, I will hone in on the English books dealing with scientific and mathematical topics. They range in complexity, from simple introductions for children to college level textbooks. All of the following links are listed alphabetically by the author's last name [and with the name of the book series in brackets wherever applicable]. Specific topics or authors can be found with your browser's "Find" function.

*Note: A handful of these books are in DJVU format. You can get a free reader for this type of file format here, or you can freely convert them to PDF here. However, be warned, some of them can get quite large after conversion!

Afanasiev & Zaikov - In The Realm of Catalysis [

Akimushkin - Ethology: Animals Do What and Why [

Alekseev - Energy & Entropy

Amelkin - Differential Equations in Applications [

Anfilov - Physics & Music

Anselm - Introduction to Semiconductor Theory

Antonov, Vygodsky, Nikitin, and Sankin - Problems in Elementary Mathematics for Home Study

Antropov - Theoretical Electrochemistry

Artsimovich - A Physicists ABC On Plasma

Asratyan & Simonov - How Reliable Is The Brain? [Covers w/ Black-and-White Drawings]

Baibakov & Martynov - Satellite & Typhoon: Eye to Eye [

Baikov & Strongin - Structural Design

Barashkov - Luminescence in Public Health [

Belikov - General Methods for Solving Physics Problems [DJVU]

Belyaev - Strength of Materials

Beskin - Dividing A Segement In A Given Ratio [

Beskin - Fascinating Fractions [

Beskin - Images of Geometric Solids [

Bitsadze & Kalinichenko - A Collection of Problems on the Equations of Mathematical Physics

Bitsazde - Equations of Mathematical Physics

Boltyanskii & Gohberg - The Decomposition of Figures into Smaller Parts [

Boltyansky - Differentiation Explained [

Borisov - Can Man Change The Climate?

Borisov & Piatnova - Harnessing the Sun

Borisovich, Bliznyakov, Izrailevich, and Fomenko - Introduction to Topology

Borovik-Romanov - Low Temperature Physics [

Bukhovtsev, et al. - Problems in Elementary Physics

Bulatov - Plugging Materials and the Cementing of Wells

Buyanov - First Aid

Byalko - Our Planet, The Earth [

Danin - Probabilities of the Quantum World

Darkov & Kuznetsov - Structural Mechanics

Demidov - How We See What We See [

Demidovich, et al. - Problems in Mathematical Analysis

Derjaguin & Fedoseev - Diamonds Wrought By Man [

Dilman - The Grand Biological Clock [

Dmitriev - Molecules Without Chemical Bonds

Dmitriev - Symmetry in the World of Molecules

Efimov & Rozendorn - Linear Algebra & Multi-Dimensional Geometry

Efros - Physics and Geometry of Disorder: Percolation Theory [

Einstein & The Philosophical Problems of 20th Century Physics [A collection of essays by many different authors.]

Elsgolts - Differential Equations and the Calculus of Variations

Epifanov - Solid State Physics

Erofeev - Progeny of Volcanoes [

Ershov & Palyutin - Mathematical Logic

Fedchenko - Things to Come [Covers w/ Moscow Star]

Fedorov - Man & Nature

Fersman - Geochemistry for Everyone

Fetisov - Proof in Geometry [

Fialkov - The Extraordinary Properties of Ordinary Solutions [

Filonovich - The Greatest Speed [

Flerov & Ilyinov - On the Way to Super Elements [

Fock - Fundamentals of Quantum Mechanics

Fock - The Theory of Space, Time, and Gravitation

Frish - Problems of Wave Optics

Gelfond - Solving Equations in Integers [

Ginzburg - Key Problems of Physics & Astrophysics

Gladkova & Kutylovskaya - Selected Questions and Problems in Physics

Glinka - General Chemistry (Volume 1)

Glinka - General Chemistry (Volume 2)

Glinka - Problems in General Chemistry

Glushkov - Engineering Methods for Analysing Strenth and Rigidity

Gnedenko - Theory of Probability

Golovina & Yaglom - Induction in Geometry [

Gordon & Gordon - The Sky Will Be Blue

Grechko, Sugakov, Tomasevich, and Fedorchenko - Problems in Theoretical Physics

Green & Shokalsky - Semiconductors: Testing & Adjusting

Gribanov - Albert Einstein's Philosophical Views and The Theory of Relativity

Grigoryev & Myakishev - The Forces of Nature

Gurevich & Chernin - The Magic of Galaxies and Stars

Gurskii - Elementary Physics Problems and Solutions

Gusev, Litivinenko, and Mordkovich - Solving Problems in Geometry

Ikramov - Linear Algebra Problems

Irodov - Basic Laws of Electromagnetism

Irodov - Fundamental Laws of Mechanics

Irodov - Problems in Atomic and Nuclear Physics

Irodov - Problems in General Physics

Ivanov - Fundamentals of Physics

Ivchenko, Medvedev, and Chistyakov - Problems in Mathematical Statistics

Kadomtsev - Plasma Physics [

Kaganov - Electrons, Phonons, Magnons [DJVU]

Kaganov & Lifshits - Quasiparticles

Kaganov & Tsukernik - The Nature of Magnetism [

Kaluzhnin - The Fundamental Theorem of Arithmetic [

Karapetyants & Drakin - The Structure of Matter

Kartsev - Three Thousand Years of Magnets

Katin - Cells, Cells, and More Cells

Khurgin - Did You Say Mathematics? [Covers w/ Moscow Star]

Khurgin - Yes, No, or Maybe [

Kikoin - Encounters with Physics and Physicists [

Kikoin - Physics In Your Kitchen Lab [

Kikoin - Senior Physics 1

Kinasoshvili - Strength of Materials

Kitaigorodsky - I Am A Physicist [Covers w/ Black-and-White Drawings]

Kitaigorodsky - Introduction to Physics [DJVU]

Klimov - Nuclear Physics and Nuclear Reactors

Klushantsev - All About The Telescope

Knorre - A Visit to Transurania [Covers w/ Moscow Star]

Kolesnikov - The Root System of Fruit Plants

Kolesnikov & Glazkov - A Spaceship in Orbit [

Kolmogorov & Fomin - Measure, Lebesgue Integrals, and Hilbert Space

Komarov - This Fascinating Astronomy [

Kompaneyets - A Course of Theoretical Physics (Volume 1)

Kompaneyets - A Course of Theoretical Physics (Volume 2)

Kompaneyets - Theoretical Physics

Kondratov - Sounds & Signs [Covers w/ Black-and-White Drawings]

Kondratov - The Riddles of Three Oceans

Kondryatev - The Structure of Atoms and Molecules

Konokov & Kiran - Vegetable Growing in Home Gardens of Tropical and Subtropical Areas

Kopylov - Elementary Kinematics of Elementary Particles [

Kopylov - Mathematical Models of Electric Machines

Korovkin - Inequalities [

Korsunsky - The Atomic Nucleus

Koshkin & Shirkevich - Handbook of Elementary Physics

Krasnov - Aerodynamics (Volume 1)

Krasnov - Aerodynamics (Volume 2)

Krasnov, Kiselev, and Makarenko - Problems and Exercises in Integral Equations

Krasnov, Kiselev, and Makarenko - Vector Analysis

Krestov, Kobenin, and Dobrotin - From Crystal To Solution [Covers w/ Moscow Star]

Krotov - Aptitude Test Problems in Physics

Kurosh - Algebraic Equations of Arbitrary Degrees [

Kutepov & Rubanov - Problems in Geometry

Kuznetsov - Einstein

Kuznetsov & Omerlyanovskii - Philosophical Problems of Elementary Particle Physics

Lakhtin - Engineering Physical Metallurgy

Landau & Kitaigorodsky - Physics for Everyone: Motion Heat

Landau & Kitaigorodsky - Physics for Everyone (Book 1: Physical Bodies)

Landau & Kitaigorodsky - Physics for Everyone (Book 2: Molecules)

Landau & Kitaigorodsky - Physics for Everyone (Book 3: Electrons)

Landau & Kitaigorodsky - Physics for Everyone (Book 4: Photons and Nuclei)

Landau & Rumer - What is The Theory of Relativity?

Landsberg - Elementary Textbook on Physics (Volume 1)

Landsberg - Elementary Textbook on Physics (Volume 2)

Landsberg - Elementary Textbook on Physics (Volume 3)

Lange - Physical Paradoxes & Sophisms

Lange - Physical Problems for Robinsons

Lekhnitskii - Theory of Elasticity of an Anisotropic Body

Leonov - The Sun's Wind

Levin & Radlova - A Picture Book of Astronomy

Levitan - A Book About Stars and Planets

Lidsky, Ovsyannikov, Tuliakov, and Shabunin - Problems in Elementary Mathematics

Lifshits - Quantum Theory of Solids [

Lipunov - In The World of Binary Stars [

Litvinenko & Mordkovich - Solving Problems in Algebra and Trigonometry

Logunov & Mestvirishvili - The Relativistic Theory of Gravitation

Luchnik - Why I'm Like Dad

Lyubich & Shor - The Kinematic Method in Geometrical Problems [

Malakhov - The Mystery of Earth's Mantle [Covers w/ Black-and-White Drawings]

Manoilov - Electricity & Man [Covers w/ Moscow Star]

Markov - Cybernetics of Living Matter

Markov - Cybernetics Today

Markusha - Miracles on Wheels

Markushevich - Areas & Logarithms [

Markushevich - Complex Numbers & Conformal Mappings [

Markushevich - Recursion Sequences [

Markushevich - Remarkable Curves [

Maron - Problems in Calculus of One Variable

Maslov - Operational Methods

Maslov & Volosov - Mathematical Aspects of Computer Science Engineering

Matveev - Electricity & Magnetism

Matveev - Mechanics and Theory of Relativity

Matveev - Molecular Physics [DJVU]

Matveev - Optics

Meshchersky - Collection of Problems in Theoretical Mechanics

Mikhailov - Partial Differential Equations

Mikhailov - The Riddle of The Self

Minorsky - Problems in Higher Mathematics

Mishchenko & Fomenko - A Course of Differential Geometry and Topology

Mishchenko, Solovyev, and Fomenko - Problems in Differential Geometry and Topology

Mitroshenkov - Pioneers of Space

Modenov - Problems in Geometry

Muslin - Machines of the 20th Century [Covers w/ Moscow Star]

Myasnikov & Osanova - Selected Problems on Physics [DJVU]

Myskis - Introductory Mathematics for Engineers

Nesturkh - The Origin of Man

Nesturkh - The Races of Mankind

Nikolaev - Space Chemistry [Covers w/ Moscow Star]

Novikov - A Planet of Riddles [Covers w/ Black-and-White Drawings]

Novozhilov & Yappa - Electrodynamics

Olgin - Experiments Without Explosions

Ostrovsky - Holography and its Application

Panchishkin & Shavgulidze - Trigonometric Functions

Panovko - Elements of the Applied Theory of Elastic Vibration

Parnov - At the Crossroads of Infinities

Parton & Perlin - Integral Equations in Elasticity

Petrov - Me or Not Me [

Petryanov & Trifonov - Elementary Order [

Pinsky - Problems in Physics

Piskunov - Differential & Integral Calculus

Podolny - Something Called Nothing

Pogorelov - Analytical Geometry

Pogorelov - Geometry

Polyakov - Semiconductors Made Simple [

Prilepko - Problem Book in High-School Mathematics

Proskuryakov - Problems in Linear Algebra [DJVU]

Pushkov & Silkin - The Quiet Sun [Covers w/ Black-and-White Drawings]

Rabiza - Space Adventures in Your Home

Rakitov - The Principles of Philosophy

Rastrigin - This Chancy, Chancy, Chancy World [Covers w/ Moscow Star]

Razmkhnin - Radar Made Easy [Covers w/ Moscow Star]

Rebrov & Khozin - The Moon & Man [Covers w/ Black-and-White Drawings]

Rekach - Manual of the Theory of Elasticity

Rosenfeld & Sergeeva - Stereographic Projection [

Rydnik - ABCs of Quantum Mechanics

Sagdeev - Nonlinear Phenomena in Plasma Physics and Hydrodynamics [

Saparina - Cybernetics Within Us [Covers w/ Black-and-White Drawings]

Savelyev - Fundamentals of Theoretical Physics (Volume 1)

Savelyev - Fundamentals of Theoretical Physics (Volume 2)

Savelyev - Physics: A General Course (Volume 1)

Savelyev - Physics: A General Course (Volume 2)

Savelyev - Physics: A General Course (Volume 3)

Schreider - Equality, Resemblance, and Order

Sedov - Entertaining Electronics

Sena - A Collection of Questions and Problems in Physics

Sena - Units of Physical Quantities and Their Dimensions

Sergeev - Physiology for Everyone [Covers w/ Moscow Star]

Sergeev - The World of the Amphibians [

Sharygin - Problems in Plane Geometry [

Sharygin - Problems in Solid Geometry [

Shaskolskaya & Eltsin - Selected Problems in Physics

Shcholkin - Physics of the Microworld [Covers w/ Moscow Star]

Shilov - Calculus of Rational Functions [

Shilov - Plotting Graphs [

Shklyarsky, Chentsov, and Yaglom - Selected Problems & Theorems in Elementary Mathematics

Shkolenko - The Space Age

Shulpin - Learning About Chemistry [

Shvarts - The Code of Life [Covers w/ Black-and-White Drawings]

Sidorov, Fedoryuk, and Shabunin - Lectures on the Theory of Functions of a Complex Variable

Smilga - In the Search for Beauty [Covers w/ Black-and-White Drawings]

Smilga - Relativity & Man

Smirnov - Faust Versus Mephistopheles?

Smirnov - Problems on the Equations of Mathematical Physics

Smogorzhevsky - Lobachevskian Geometry [

Smogorzhevsky - Method of Coordinates [

Smorodinsky - Temperature [

Sobol - The Monte Carlo Method [

Sobolev - Lasers and Their Properties

Solodovnikov - Systems of Linear Inequalities [

Solodovnikov & Toropova - Linear Algebra with Elements of Analytic Geometry

Sominskii - The Method of Mathematical Induction

Spiridonov - Universal Physical Constants

Spitsyn, Vorobyova, Dunaeva, Ippolitova, and Tamm - Practical Inorganic Chemistry

Srebrodolsky - Mystery of Mineralogy [

Starzhinskii - Applied Methods in the Theory of Nonlinear Oscillations

Steinhaus - The Nine Colors of The Rainbow

Stepnova - Fun With Numbers

Sveshnikov & Tikhonov - The Theory of Functions of a Complex Variable

Tamm - Fundamentals of The Theory of Electricity

Tarasov - Basic Concepts of Quantum Mechanics

Tarasov - Calculus: Basic Concepts for High Schools

Tarasov - Laser Age in Optics

Tarasov - Laser Physics and Applications

Tarasov - Laser Physics

Tarasov - The World is Build on Probability

Tarasov - This Amazingly Symmetrical World

Tarasov & Tarasova - Discussions on Refraction of Light [

Tarasov & Tarasova - Questions and Answers in School Physics

Targ - Theoretical Mechanics: A Short Course

Targ - Theoretical Mechanics

Tomilin - How People Discovered the Shape of the Earth

Trifinov - 107 Stories About Chemistry

Trifonov & Trifonov - Chemical Elements

Trifonov & Vlasov - Silhouettes of Chemistry [

Tsytovich - Soil Mechanics

Ugarov - Special Theory of Relativity [DJVU]

Umansky - Everyday Neuropathology

Uspensky - Godels Incompleteness Theorem [

Uspensky - Pascal's Triangle [

Uspensky - Post's Machine [

Varshavsky & Pospelov - Puppets Without Strings [

Vasilyev - Metals & Man

Vasilyev & Gutenmacher - Straight Lines & Curves

Vasilyev & Stanyukovich - Matter & Man

Vasyuchenko - Problems and Exercises in Chemistry

Venetsky - On Rare & Scattered Metals

Venetsky - Tales About Metals

Venttsel - Elements of Game Theory [

Vilenkin - Combinatorial Mathematics [DJVU]

Vilenkin - Method of Successive Approximations [

Vladimirov - Generalized Functions in Mathematical Physics

Vladimirov, Mitskievich, and Horsky - Space, Time, Gravitation

Voitkevich - Origin and Chemical Evolution of the Earth [

Voitkevich - Origin and Development of Life on Earth [

Vonsovsky - Magnetism of Elementary Particles

Voronov - Storming the Fortress of Fusion [

Vorontsov-Velyaminov - Essays About The Universe

Voyevodin - Linear Algebra

Vygodsky - Mathematical Handbook

Wentzel - Operations Research

Wentzel - Probability Theory

Wolkenstein - Electrons & Crystals [

Wolkenstein - Problems in General Physics

Yaglom - An Unusual Algebra [

Yasamanov - Modern Geology [

Yavorsky & Detlaf - A Modern Handbook of Physics

Yavorsky & Pinsky - Fundamentals of Physics (Volume 1)

Yavorsky & Pinsky - Fundamentals of Physics (Volume 2)

Zeldovich & Myskis - Elements of Applied Mathematics

Zeldovich & Yaglom - Higher Mathematics for Beginners

Zelkin - Flying Trains [

Zhitomirski & Shervin - Let's Play Geometry

Zhitomirski & Shervin - Maths With Mummy

Zigel - Wonders of the Night Sky

Zubov & Shalnov - Problems in Physics