Is this because people seek knowledge from LLMs rather than from books now?
Or is it because LLMs know everything that is in books, so people don't feel compelled to learn any more themselves?
even before LLMs you could/can get a lot of info on the web. Most ideas presented in books exist on the internet somewhere, quite a few pass through HN. In the 90s and early 2ks I used to hoard books, but now, not so much, I get a few through my library these days but a lot of times I just find the book is padded out to be a book and the meaty bit of the book is usually tiny.
Another option might be that people are increasingly using LLMs to write the books.
Definitely isn't the latter. Numerical Recipes and Hacker's Delight have tons of gems that you won't get from an LLM, or that an LLM will even understand despite appearing all their training sets.