Unless this is a book the author wrote solely to enjoy themselves, let's not pretend that feedback from potential buyers is something that shouldn't be allowed.
The author has responded to this feedback on the website in great detail. At this point, this is less of feedback and more of entitled people demanding that the author write another book.
Professors frequently write their own textbooks. Sometimes these are only used in their own classrooms. That's pretty close to their own enjoyment. It allows them to have a book that exactly matches the way they want to teach. As far as buyers, this book is free.
