Soviet era Mathematics (and other sciences) books here : https://mirtitles.org/ These are concise but quite dense and pretty good. Students are well advised to go through these in addition to their prescribed textbooks.
A simple search and bibiography might add a lot of value here
One of us who is enthusiastic enough might implement a "ask this book" chatbot on top of this repo, maybe? Hopefully that doesnt violate any copyrights either