I bought a copy of The Fellowship of the Ring at Halfpriced Books, and it still had a Christmas card in it, from a son to a father. It was the sort of card you wouldn't want thrown away (or sold). I think I kept it, though now I'm not sure where.
I once left a postcard bookmark in a UL book when I returned it. It was from my Hungarian teacher, in Hungarian, saying good luck for my A-levels. Fortunately the next person who borrowed the book knew me and guessed it was important.