Not really a resource for textbooks (unless, I guess, you're in English or Classics or something), but I love listening to Librivox. They've got tons of audio files of books read by volunteers up for free. It's really nice for when you're doing something mindless (stockinette), especially to a deadline, when you have lots of time to listen to something the whole way through.
Otherwise, I just listen to the TV/movies. Nothing subtitled, though... that doesn't work so well.