"All the people who borrow my books and don't return them don't even read my blog. Note to those people: Fuck you, you fuckers!"
I feel you Doppelganger.
My trick is just to have a really bad memory so I never remember which books I own, which I got from the library, which were loaned to me and which I loaned out. If someone reminds me that I have a book of theirs, I certainly give it back and if I do remember that you left my life with one of my books, well then, a pox on you!