How To Solve It by G. Polya https://press.princeton.edu/books/paperback/9780691164076/how-to-solve-it https://press.princeton.edu/books/paperback/9780691164076/

How To Solve It by G. Polya https://press.princeton.edu/books/paperback/9780691164076/how-to-solve-it https://press.princeton.edu/books/paperback/9780691164076/ho... A Logical Approach to Discrete Math by David Greis and Fred Schneider, https://link.springer.com/book/10.1007/978-1-4757-3837-7 https://link.springer.com/book/10.1007/978-1-4757-3837-7 I'm self-taught so for me it was learning how to write proofs that gave me a big boost in being able to branch out into different area of interest and not give up. :)