From the ShowHN guidelines: https://news.ycombinator.com/showhn.html

> Show HN is for something you've made that other people can play with. HN users can try it out, give you feedback, and ask questions in the thread. [...] For example, blog posts, email signups, and fundraisers can't be tried out, so they don't count as Show HNs.

Books are a difficult type, because it's not possible to "try" it online. Someona asked about this, and dang answered: https://news.ycombinator.com/item?id=8765822

> The "try out" rule is important, but adjusts to different kinds of project. Hardware, for example, can't be tried out as easily as software. For a book, I suppose a sample chapter or two would be a fair way of trying it out.