I can relate to the part about not remembering who it was that mentioned a book that you wound up ordering. In my case it isn't books but jourmal articles. When I realized that I had many that I couldn't remember why I had downloaded, or when, I added a field to my citation tracker database saying which date I downloaded (or put in an ILL request), and why (cited by another paper? Found it while searching for a key word? Emailed to me by a collegue? Plus any other info my future self would want.)