The question that I think you should be asking is which field of mathematics to learn next. You have high school knowledge, you want to learn how to read and write proofs, I personally believe the right place to go is number theory.
Within number theory, different books and different learning styles are good for different people. I personally like to get a book, see if I like the topic, and if I do then get another book so that I can compare them. You might try one of these elementary books on number theory:
Combinatorics and graph theory are reasonable too, but IMO it is too easy to learn aspects of those subjects without really biting off the "how to handle proofs" thing. Analysis and abstract algebra make more sense after number theory first - number theory is basically "pre abstract algebra".
Within number theory, different books and different learning styles are good for different people. I personally like to get a book, see if I like the topic, and if I do then get another book so that I can compare them. You might try one of these elementary books on number theory:
https://www.amazon.com/Elementary-Number-Theory-Second-Mathe...
https://www.amazon.com/Introduction-Number-Theory-MIT-Press/...
https://artofproblemsolving.com/store/book/intro-number-theo...
Combinatorics and graph theory are reasonable too, but IMO it is too easy to learn aspects of those subjects without really biting off the "how to handle proofs" thing. Analysis and abstract algebra make more sense after number theory first - number theory is basically "pre abstract algebra".