I read incredibly fast. Reading a book a night is no problem for me. I think it is genetic, as my mother could read at a pace that astounded even me, and she could recall everything from the book too. A 500 page novel would take her about 2-3 hours to get through, then she could recite the book back to you - scenery, dialogs the works.
Now this is talking about novels. If I'm reading to learn then you have to I must interact with the book, stop and think, go back and make connections, try something out etc.
This is where I like the statement in the book "How to read a book" about reading as fast as is necessary to complete the task you want to accomplish - fast for a quick read through maybe, then slower on another read through for making notes.
So I can read a novel a night, or a pop Sci book in a day, but I've been going through a math book for the last 2 months, and of course, I've been reading the first chapter of TAOCP for about 15 years (hence the math books now :)