Lately, i have try to find a answer for this question. I will be glad if some of you would point me to the right literature. With that being said, i will tell you why i ask this question. I have attended numerious courses in mathematics, and physics, and the one thing i have noticed is that most professors( and the student) felt that the book in physics and mathematics are bad. Perhaps some student might be able to skip by in lower division courses without attending the lectures, but it becomes a bit difficult to learn say the method of proof by reading the book. It is often more helpful to study the class notes. My question is: if the notes are so much useful than the book, then why are there so many books? My professors answer is that most professor are too lazy to write it.