World of Mathematics - An amazing compendium of accessible content straight from the masters - Poincare, Jonathan Swift, Neumann, Bishop Berkeley, Cayley etc https://www.amazon.com/World-Mathematics-James-Newman-Hardco... I believe it is available on archive.org

What Is Mathematics? by Richard Courant and Herbert Robbins published in 1941. One of the most beginner friendly yet rigorous books out there for a survey of many areas in mathematics.

You can find hundreds of gems here from erstwhile Soviet Union - https://mirtitles.org/