Haven't tried one, but it seems like it would work better than a book. Books are awkward to me, because you've got to hold them, which means keeping body parts outside the sleeping bag, and lighting is still an issue to deal with.
I have used an MP3 player and audiobooks, which most libraries have for free download. Maybe not ideal if you need to keep your ears open though. In more crowded campgrounds, the player can help mask noise from rude campers or late-nighters. And once, I woke up around 3AM and was wide awake for some reason. I ended up watching Darjeenan Express on the player's 3" screen, and by the end I was nice and sleepy and dozed right off. Not exactly a backcountry experience, I guess, but who cares, it worked and I enjoyed it.
Gary
Yosemite Photo Galleries: http://www.pbase.com/roberthouse/yo