I've written two books, both well over 100,000 words. While writing the books, I had to read, and re-read them several times, of course. I also spent quite a bit of time researching things, reading parts of other people's books.
The first one I wrote before ChatGPT existed. I finished the second one last year, but I have no idea how to use ChatGPT to write a book, so I didn't try. Besides, writing is fun, why let a stupid machine do it?