first, you should be able to handle
headings of different levels, such as
the book, chapters, and subsections.
you may label the levels as you like.
html can support 6 different levels, so that's a good number to shoot for.
one of the things that users find handy is a table of contents for the e-book, so you must be capable of generating one, in cases where an e-text doesn't have one.
because of their experience with the web, people often expect this table of contents to be hotlinked to the appropriate sections, so your markup system should facilitate that. a nice touch is then to have chapter headings then link back to the table of contents...