Seriously, though. I've loaned out lots of books over the years and most of them, I've never seen again. I usually just buy another one, if I really want the book, and consider the lost one a gift.
To try to encourage people to return my books, I bought some book tags, which are like stickers you put inside the book that says "this book belongs to ______________."
So about two weeks ago, I loaned a co-worker
The Five Love Languages. The next day, it was still sitting on his desk, so I fussed at him a little bit (he and his wife DEFINITELY speak different languages, and it has caused a lot of problems, and he really wants help). So yeah, today, I noticed the book is still sitting in the same spot on his desk. I'm concerned that he's not going to actually read it (though he expressed interest when I told him about the book), and that after a while, it'll get lost or misplaced or thrown out or 'borrowed' or whatever - and I'll never get it back.