Strypey if you think of your book as code, like md, latex, rst or asciidoc you can use any code repository to collaborate, like these that Liam recommended.
You can use something like pandoc or mdbook or astro to turn your book into html or pdf or whatever. I'm liking mdbook and Astro (depending on target audience and book features). Every edit appears on a Web URL in seconds.
Asciidoc and asciidoctor is good too. I just hate Ruby.