This is so that people can learn. We're not favoring any book, we're just telling them "this is a good book, you can learn from this". To me at least, the official docs aren't that good. They take a long time to gain pace. It's too long a read, and if from the new contributor's perspective,  (s)he doesn't want to pore over long articles, (s)he just wants to get into contributing faster. The listed books are quick and good in explanation as far as I've read. So imo, we should keep this.