I started reading a blogger who teaches how to make a successful blog, sells courses about it, etc. I soon realized her blog is just about her making money from promising to help you make money from your blog. A frighteningly vacuous operation. It seems a modern cousin to the old ad in magazines promising to reveal the secret to wealth for $20. When you write to the given address you get a letter back saying "Do as I do." It's a pyramid operation.
Writing books about writing books seems similar. Selling the dream of high sales when you write on other topics, when their books are the same "making money from promising to help you make money" from your book seems a bit fishy, has a scammy element. It's more than a lil "Do as I do."