
I thought everybody knew of this site especially Indians since it was started and is lovingly maintained by an Indian :-) A lot of hard work has gone into it with not just scanning of the books but also typesetting some of them in latex for better rendering and reading. The website author deserves all the credit/acclaim he can get.
All of the books are hosted on internet archive which you can browse here - https://archive.org/details/mir-titles
Many of the titles are also published in hard copy form by low-cost Indian publishers on Amazon India.
Some recommendations which are not typical textbooks;
1) Little Mathematics Library series - https://mirtitles.org/2024/05/11/little-mathematics-library-...
2) Books by Lev Tarasov. Some of these books are structured as a dialogue and thus walks you through the conceptual process. Titles are on Calculus, Probability, Quantum Mechanics, Physics Q/A etc. All worth reading; some of them are latex remastered versions - https://mirtitles.org/?s=Tarasov
3) Higher Math for Beginners(with Yaglom) and Elements of Applied Mathematics(with Myskis) by Zeldovich - https://mirtitles.org/?s=zeldovich
4) Mathematical Handbook (two vols); Elementary Mathematics and Higher Mathematics by M.Vygodsky. - https://mirtitles.org/?s=vygodsky
5) Handbook of Physics by Yavorsky and Detlaf - https://mirtitles.org/?s=Handbook+of+Physics
6) Fundamentals of Physics by Ivanov - https://mirtitles.org/2018/04/21/fundamentals-of-physics-iva...
7) Ever popular Science/Maths books by Yakov Perelman viz. Physics for Entertainment, Mathematics can be Fun etc. - https://mirtitles.org/?s=Yakov+Perelman
A group of people decided to seat together and talk about some casual math problems
The one problem I keep remembering is a bet about 1000 men walking by on the street in a row. Random chance is not guaranteed - especially when it's suddenly a parade :)
Specifically; start with the 2-vol Physics for Entertainment (the breadth of coverage is really good) followed by Mathematics can be Fun, Algebra can be Fun etc.
Note: Do not be misled by the words "Entertainment"/"Fun" etc. in the title. As is typical of Soviet era books these are serious works with precise, succinct and focused topics quite unlike most current "pop science" books.