Richard Hammack has written an excellent OER textbook, Book of Proof, which is designed to teach students the techniques of proof writing. It boasts an intuitive organization, polished format, and useful exercises; it comes equipped with a helpful webpage and many ancillary materials that other OER textbooks lack. Other textbooks in this category that deserve attention are Mathematical Reasoning: Writing and Proof by Ted Sundstrom and A Friendly Introduction to Mathematical Logic, by Christopher C. Leary and Lars Kristiansen.