I've enjoyed reading murder mysteries while travelling. Anything by Agatha Christie is good. Generally speaking short stories,essays or something with a strong narrative flow makes for a good travel book.

Another good idea is to make yourself read a book that you would never have read at home. While travelling, if you don't have other distractions(internet,music etc) and only a book to keep you occupied (when you are not busy sightseeing) you might actually finish it. I read The Republic in two days on a train-- a book which I wouldn’t have touched otherwise. And I am glad I did :) It made me discover a completely new class of books.