OpenOffice.org also has a powerful native PDF export function, one with a greater range of options than the native exporter in Microsoft Word. That's useful as long as you don't mind using PDF as a target document type.
A more modest example of an e-book production application, Sigil is both free and open source. It's a lot closer to an editor that exports to e-books (it sports a built-in document editor) than a conversion suite for existing documents, but it also includes various tools for collating and assembling a finished e-book (such as a table-of-contents editor).
Sigil's main drawback is how it handles importing. It only accepts HTML, plain text or existing ePub files as input documents, so it will most likely work best if you are able to export your original document to HTML in a way that preserves all of the most important formatting. A similar program, Jutoh, accepts OPL files and has slightly more robust editing options; it costs $39.
The recent massive surge in demand for e-books hasn't yet triggered a concomitant surge in development of polished products for e-book production. The one thing that's most conspicuously lacking is a single gold-standard product that guides users through the whole workflow and helps them check their results. With all the different book formats that are floating around, putting together such a product might well be an order of magnitude tougher than anyone expects.
The good news is that the e-book boom has helped consolidate formats a bit. The Kindle, the Nook and the iTunes Bookstore (which services both the iPhone and iPad) now stand out as the most common targets for e-books.
The time's right for a product that can walk you through the whole process. For now, though, we'll have to settle for using the tools that do exist, and using them with care and attention.
Sign up for Computerworld eNewsletters.