Generally, it tends to be books that one can just dip into - books containing lists or other factual books, encyclopedias or pub quiz trivia. Having said that, I don't think I ever went to the bathroom in my grandparents' old house without there being a Farside Gallery book, a Calvin & Hobbes book and the newest edition of Private Eye magazine.
My maternal grandparents' house only rarely contained a magazine or newspaper, but on the back of the door was a huge poster detailing the history of the British monarchy, so that was more than enough information to attempt to absorb. I love the idea that bathroom time shouldn't be wasted time - that it should be used as a further opportunity to learn!
The walls of my own WC are currently covered in posters - one details cloud names, types and formations, another pictures shows British birds, another the flags of the world and the last is a detailed map of the moon. A nice selection, I thought, and rather necessary since a lot of people think it odd to keep books in there. Despite this, there is also a book of British words and sayings and yet another history of the British monarchy. Quite a theme there, methinks.
How about these immense bathroom libraries! Jealous much? I certainly am!
What do you have in your bathroom? Any books?