I've written an online book with mdbook (https://lhbg-book.link) and it was a breeze. I believe mdbook was one of the reasons I even finished the book.

1. It's super easy to install. If you have a rust toolchain, just `cargo install mdbook`

2. One command to initialize: `mdbook init my-book`

3. One command to get immediate continuous feedback: `mdbook serve`

4. It allowed me to keep writing in my preferred environment (emacs)

5. It looked good by default. I could focus on the content.

6. Setting up auto deploy ci on github is about 30 lines for yaml

Though one point of improvement would be better support for other export formats such as pdf and epub.

tl;dr mdbook allowed me to use the path of least resistence to complete my project, and I highly recommend it.

Also it is possible other platforms can do the same or better but I haven't tried them.