It is really no good to our health when you go to the toilet and read the book which is bad for your digestion system. However, if the light is not bright in the toilet, reading books may be bad for your eyes. If you do this often, your eyes may get short sighted. In a word, you'd better not read while you are on the toilet.
In my opinion, this problem is quite interesting. I do know that there are some people who will do just like you, anyhow I would never do that. Indeed, the first is taste, which can do harm to your body. The second is light, usually, toilet is not open, so I am sure that it is bad for reading. If you go on for a long time, it will undoubtedly affect your eyesight.
As far as I'm concerned, more and more people like to do such a thing..reading while on the toilet, including me. Because I think it is really a kind of way to relax and I can easily concentrate in reading while I'm on the toilet. In most cases I think, it is harmless for us to do some reading while on the toilet.
However, I was told that there also a kind of possibility of hemorrhoids.
It is said in some studies about the development of hemorrhoids with prolonged toilet time, that blood return from the ass hole area is impeded during prolonged sitting, and even if you're not pushing all the time, the position you're sitting in with your cheeks apart still allows more pressure on the rectal veins which will lead to engorgement of ass veins and creation of hemorrhoids.
Though, I still love to reading book or magazine on the toilet.