Unfortunately I wasn't really that interested in math when I was a student, I just remembered the ranting of my math teacher. I think you can still get quite a few on Ebay but I fear that not much has been digitized. I can recommend a few Soviet works however that are more introductory level and that I have worked with myself.

A good example of a introductory book with good didactic is Probability Theory (first steps) by Wentzel. https://archive.org/details/ProbabilityTheoryfirstSteps/mode...

Also Physics of Everyone, though maybe a bit more difficult: https://archive.org/details/LandauKitaigorodskyPhysicsForEve...

Link to all the Mir titles: https://archive.org/details/mir-titles

There are books for basically every level and age there.