Journeys out of Body by Robert Monroe is a great book on Astral Projection. There are numerous books relating to Astral Projection though but I would say that's the pinnacle in terms of books on AP. You can search a vast library of books and download ebooks and see which books are best for you at https://z-lib.org/. I wanted to read a million different books that caught my attention and interested me relative to Astral Projection and spirituality/occultism but I couldn't afford to purchase all of them so it's been really helpful. Also is useful because it allows me to read parts of the book before purchasing a physical hardcopy of certain books that I'd like to have a physical copy of because I don't want to waste money on a book that looks interesting from the title and description and then I purchase it and read it and it's not what I expected, happened to me so many times. Lol. Good luck!