Fixed now. That was a bug in our software - sorry!
Btw, for a Show HN that's a book, the convention is to provide a sample chapter (or something similar) for free. Otherwise there's no way for users to "try it out", which is the necessary condition for a Show HN: https://news.ycombinator.com/showhn.html.
I’m trying to change it but apparently it’s not possible to adjust URLs after submission?
Hoping one of the mods could help with this!