[buixuanquy]

The best book about proof I've read is "How to prove it" by Daniel J. Velleman. A lot of set theory. But to understand it, you must do the exercises in the book. And for the solution, checkout this blog: https://www.inchmeal.io/htpi/index.html

[Jimpulse]

Thanks for the recommendation and solutions. It's the second one in this thread. I'll definitely be purchasing it.