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.