>The book contains no advertisements.

I think the blog post is poorly thought out and I understand your point. Nevertheless, many books have "ads" in the sense of "read more books by this author" on the inside dust jacket of a hardback, or placed in the last few pages of a paperback. Sometimes when I buy a book directly from the publisher, they will include in the box some brochures/catalogs for similar books. The theme for all the situations is the same -- they already have your eyeballs tuned in to their wares to some extent -- so they try to take advantage of it.

It's just that the ads are not wedged in between the middle of the book to interrupt the reading experience.