Book formatting is very much a matter of taste (although one should always consider accessibility of course), so we actually don't really offer any formatting advice, other than the fact that we do have default settings for the Nonfiction, Fiction, and Technical book themes.
You can see examples of all of the fonts here:
Basically our position is, when you're writing you shouldn't think much about formatting, and when your book is done, play around with the book theme settings until you get the result you like!
Also, when you're done writing, you can also hand off your book to a professional designer and then switch your book to being in our Upload writing mode, which lets you upload your custom ebook files.