Taking some reading material with you to pass the time on the john is one thing; setting up shop in there is another. A bathroom is not a library, nor is it a cafe. So while you might peruse a few pages while handling your business, polishing off an entire novel is a bit excessive.
For one thing, it's not healthy. Doctors say too much time in the commode can contribute to hemorrhoid growth. If the anal cushions (yes, that's the anatomical term) become stretched, they can cause bleeding, itching or pain [sources: Fennessy, Esquire]. For another thing, a marathon trip to the stall can really cramp the style of anyone who may be waiting to use it. Do yourself and your fellow man a favor: Get in and get out. Just not before you check that the seat is down and dry.