Recently, I loaned my favorite book of all time, an out of print book about backpacking through the Golden Trout Wilderness fishing for trout, to a younger lady I trusted. It never came back at all.
I have loaned out countless books that I've never seen back. It's hard for me to say "no" to loaning someone a book, especially if it's a good one. Knowledge needs to be passed on IMO. I do have a few books with emotional/sentimental value that don't go out, though.