What are good books on proofwriting?

Question

I wondered about some good books that aren't necessarily "introductory", but delve deeper into the "art of proof-writing".

Answer 1

I appreciated reading this one, very well written, but at an introductory level. Being simple allows the reader to focus on the main purpose of the book, which is how to efficiently combine all available information to proof or reject a proposition:
Daniel J. Velleman, 2006, How to Prove it: A Structured Approach: Second edition, Cambridge University Press 978-0-521-86124-3.

EDIT: I just saw that your question is related to this one: Book for proof writing

Answer 2

On my side I enjoyed Mathematical Writing from Franco Vivaldi. English is not my mother language and the book was very useful.