Euclid's Elements teaches geometric concepts that you're probably familiar with, but with interesting proofs: https://en.wikipedia.org/wiki/Euclid%27s_Elements

Euclid's Elements teaches geometric concepts that you're probably familiar with, but with interesting proofs: https://en.wikipedia.org/wiki/Euclid%27s_Elements https://en.wikipedia.org/wiki/Euclid%27s_Elements It's free, it's time tested, and probably just the right amount of rigor to get your proof abilities up. I think the ability to prove theorems and the ability to construct abstractions go hand-in-hand.