why would you want some internet user to write up a post instead?

A group of internet users could discuss an existing book or a group of books, and say for example: "this part worked for me", "this part didn't work for me", "I did this meta action to not forget using this part", "here is a research that disproves one of the assumptions in the book" etc. They don't have to replace the books, just build on them further.

Seems to be that many books are optimized (more or less successfully) to be bestsellers. A book that actually changes your life will not necessarily be more popular than a book that impressed you and makes you recommend it to your friends, even if your life remains unchanged or if the only change is being more (falsely) optimistic about your future successes.