@forkDestroyer I'm no author or writer but personally I would lean towards something markdown based, since it's really easy to write with.

mdbook is nice, though it's tailored towards writing documentation rather than actual books.

I do use obsidian (disclaimed: not open source) to manage notes for my D&D games. You can imagine it like writing in a wiki style, where you reference other files all the time and create a web of them. Great for worldbuilding.

If you want to make something for print then typst is something look at. I'd call it a spiritual successor to LaTeX, so you can make some very nice looking PDFs.
Good for scientific papers and the like as well.
I've also seen people make good looking slides for presentations or talks.