This book is distributed on the “read first, buy later” principle. I strongly believe that paying for the content before consuming it is buying a pig in a poke. You can see and try a car in a dealership before you buy it. You can try on a shirt or a dress in a department store. You have to be able to read a book before paying for it.
The read first, buy later principle implies that you can freely download the book, read it and share it with your friends and colleagues. If you liked the book, only then you have to buy it.