I do, but I really try to only read for as long as is necessary to take a shit, any more is bad for your health.
Well, reading isn't bad for your health, but sitting on the toilet in 'the ready position' and straining for any longer that is necessary can lead to haemorrhoids.
In a nutshell you can become so engrossed in a book that you don't realise you're doing the shitting motions for longer than you need to and begin to push your lower intestine out. Obviously this doesn't happen in one sitting, but it's a possible consequence of such prolonged and repeated behaviour, so reading on the toilet can be a bad habit to get into.