I grew up taking books to the loo with me, mainly because in our house, if you put a book down it either got covered up with something or someone else was wanting to read it and it therefore disappeared.
The last option is why my dad slept with 3 Harry Potter books under his pillow for 2 weeks, to make sure he got to read them all the way through with no interruptions! My sisters moaned like hell, but I didn't, I was a sneaky child. I went and sat on the bed, read each book in turn and put them back! Dad never guessed.
We grew up without a tv, and almost every wall was bookshelves. I can remember the year I turned 11, between the 5 of us in the house, I think there were no less than 20 books being read at any given time. Of course, with that many of us in the house, sometimes the loo was the only place you could get peace and quiet long enough to finish reading a book!
In our downstairs loo, I have a couple of half-completed crossword/codeword books, and a couple of 'chick-lit/beach-read' books that I don'tr care if they get ruined, in case someone wants reading material whilst they're in there. The bathroom upstairs has no space for books.