Hello,
Formatting depends on the type of book.
For fiction, any of the popular markup languages will do it for you as few fiction writers need much beyond bold and italic. Leo is well integrated with rst and I taught myself to use it while I was learning Leo.
For non-fiction, the more sophisticated options might be required. rst/Sphinx, asciidoc, or latex would all look after most use cases.
The autosave plugin can be modified for a short interval, although I tend to live with 300 seconds.
I use a directory watch utility in linux and commit every saved .leo file (which I keep in one folder and sync with Mega cloud storage) to a fossil repository (also synced).
HTH,
Chris