docx is technically proprietary, but published and open access - lots of different non-Microsoft software supports it. There are even a number of java libraries, some free and some commercial, for dealing with it.
Here's one.
Here's another.
But if not docx, maybe rtf?
Right now I'm exporting the book as html, hauling it into LibreOffice, editing it to insert page breaks between chapters, and then saving it as docx. Having to do a manual edit is an efficiency-killer and is likely to lead to the "same" document in different formats being edited separately - and thus having different contents.
(By the way, with my current story-in-progress, the result of Export - Book as HTML is nearly 80K in size. Export - Other Formats gives me the following sizes: HTML 589 bytes, XML 89 bytes, TXT 0 bytes.)