without normalizing for number of distinct HN users, as well as the possible incorporation of a new demographic to the site's readership that is less interested in books, but has always been present somewhere in the world, this trend doesn't actually say much. It says something about the website, news.ycombinator.com, but that isn't terribly relevant to anything.
The new demographic is probably vibe-coders, actually. These were people that were never much into reading. But because HN is generally AI adjacent, a person starting to vibe code or otherwise use AI tools will probably find HN in the sources and start using it because it shows up so frequently in forum listings.
So "WE" are probably still talking about books the same amount (pre LLMs-going-mainstream HN users), and the book trend is more of a latent eternal-September effect.