every single customer has different perspectives. And I know every perspective want must respect, how is this possible?

I recently bought the book "Think and Grow Rich" by Napoleon Hill at a well-known bookstore for $4. I absolutely do not understand how this is possible, because I have to say that it is probably the best book I have ever read. And it was only $4, WTF?! So, what's the most powerful book you have ever read?