Check out https://typst.app/ if you're looking to write a book yourself.

https://hypermedia.systems/ was written with it.

It was a mix of markdown, liquid, scss and ruby scripts via a Rakefile to tie it together to generate html.

I bought a prince XML license for PDF generation. It was a hodgepodge of stuff.

For very long documents Word can be very difficult to maintain. Anything that requires you first write in simple text and then compiles is preferred. I tried Skrivenr, but found it old, clunky, buggy and poorly designed. I was far more productive in LaTex, but encountered problems when converting to Word (which most publishers prefer). Sure I tried Pandoc, but maintaining flow between versions of my book was a small hell.

In the end, I believe that there is no easy solution.

Typst has excellent collaboration tools. Both Typst and Overleaf do not support Doc export, but pdf to doc is relatively easy.

I'm writing my book in Markdown files in IntelliJ and pushing it to GitHub. Use the tools you know.