Used bookstores and trade-a-book places are the way to go for travel books. I only take paperbacks on the road, as I need to consider both space and weight in the luggage.

If I finish a book while traveling, I will often give it to someone, or trade it in at a local used bookstore something else. Outside of the U.S. English language paperbacks are a very welcome gift - once I left a half dozen of them at a resort in Mexico, and I heard later that they had been passed all around the staff as books were a rare and welcome treat for them.

I also save up the big Sunday crossword puzzles, as I rarely find time at home to do them. They take up no space at all, and a couple of them can keep me busy for a whole flight.