I think the author mistakenly assumes that the number of books on a topic is somewhat proportional to its complexity level. It's actually inverse - how many books do you have on advanced calculus vs. how to tie a tie?
People write books to make money. The easier a topic, the easier is to write a book on it. The more popular a topic, the more profitable is to write a book on it. Hence, you get most books on things that are cool and relatively simple.
People write books to make money. The easier a topic, the easier is to write a book on it. The more popular a topic, the more profitable is to write a book on it. Hence, you get most books on things that are cool and relatively simple.