Comment by pigpop

That's not surprising, I've used LLMs to write several chapters of a book and it's many times more difficult than getting them to write working code. It's not a task that they're well optimized for but the biggest hindrance is that there are practically no tools that allow you to validate prose especially at the lengths required for a book.

Getting a good writing style out of them requires careful prompting and many corrections, their default writing style(s) are so highly reinforced by training that they will always tend to drift back to them. Maintaining continuity requires you to create a lot of documentation outside of the text itself. It's a much more manual process than working on a codebase where you've set up a lot of automation and tooling that allows them to check their own work.

Edit: it's also worth noting that many LLMs have gotten much worse at writing prose as they have gotten better at writing code.