It's ok to make a Show HN out of a book, as long as it's a real book and users can 'try it out' by reading part of it for free.

https://news.ycombinator.com/showhn.html

Hey @pl0x, the reason why I wrote this book is to help people in their tech careers. Sorry if I broke some rule or anything. I thought that it would be nice to get some feedback about the first chapter and the site.

What's wrong with that, as opposed to promoting your app?

its literally marked as "Show HN"