I was trying to figure this out for a piece of non-fiction which isn’t technical. I wrote it in Emacs using markdown and it proved to be hard to collaborate and get feedback from non-technical people. I looked at Dropbox Paper and Notion since they also supported markdown and those were also hard to get suggested changes. I looked at Word and document sharing and although that was better it had a few problems with OneDrive file sync even though I really liked the Office app on the phone. Finally, I ended up in Google Docs. It is simple, I can collaborate and it syncs readily. Also, the sidebar table of contents is nice to jump to different parts of the book easily. Sure not markdown but for my current needs since I’m not putting any code in the doc it is totally fine. To be honest I don’t know why I went through all the hurdles. Once it is closer to done I may move it to a markdown file again.