The book seemed rather short for the price. Definitely had useful information that helped me to understand issues I have with code design. It wasn't worth $50 to me, but may be for other people.
It's worth noting that the book is incomplete, chapters 5 & 6 are still being written. If I'm able to complete the book in the next day or so I'll post a follow-up.
The information is plainly stated in a way which is so simple it's novel.