Reference textbook about proof techniques I am looking for some good recommended reference textbooks about proof techniques.
Someone told me "G. Polya - How to solve it" is kind of standard, but quite old.
I am looking for a book that handles both classical (manual work) proofs and modern proof techniques using proof assistants or automated theorem provers. Or two separate books each handling just one of these topics.

I personally recommend Problem Solving Strategies
and Putnam and Beyond.
These are math-olympiad oriented. But I think they are excellent sources and they include loads of examples that are certainly not your average textbook problem.
