I started selling a "How to Book" in the early 2000's. It changed my life! Not only did I sell a hell of a lot of books at $300 - $500 each, but it put me on the front page of the Wall Street Journal, The New York Times... This led to more book sales, positioned me as the go to guy in my niche, and enabled me to collaborate on related business deals with some very savvy business people.
What did my book do for its buyers? Enabled them to realize they could duplicate my efforts and in a LOT of cases, improve upon, scale and profit even more handsomely than I managed to do.
Paying for GOOD INFORMATION that can jump start your business is always a good idea. The ability to scale your enterprise by tapping into the knowledge and experience of entrepreneurs who have come before you is a tremendous advantage. But, perform your due-diligence. Can you call the expert(s) and explore their background, experience and motivation?
Be aware that selling a book "giving out helpful/useful money-making knowledge" makes sense for authors in a plethora of ways! By making a book available, in addition to providing a repository of additional resources and updates, the author is in a position to receive in-bound calls and questions from potential buyers of a "How to Manual," a "Bible" a "White Paper..." Literally, a dialog begins and both parties are in a position to explore and learn.
Sure, skeptics say, "If you the author is so smart, why sell your book? Just put your head down and do the work!"
Selling a GOOD book opens doors! Buying knowledge is one of the smartest investments we can make!!