I hear this argument pretty often.
A book is not a binary thing: there is a book, and there isn't. It's not about the "book", it's about the contents of the book.
So in other words the author has gathered some knowledge and he didn't make much money off of it. Therefore, the knowledge you will gather will not allow you to earn any money. Does that make sense? I don't think so.
I will probably be downvoted, but maybe more important thing is the subject of the book, than the actual book itself?
Disclaimer: I did not write a book. I did write a chapter to a book, and I got some money off of it. I'm not rich because of it, but I see the potential.