A question for authors: Are there word doc formats that can be versioned (i.e. SVN) and worked on collaboratively?

I'm starting to work on a book writing project with at least one other
co-author. I'm hoping to find tools that are better than token passing -
i.e. emailing the files back and forth.

I hoped .docx would be useful but there is so much binary gunk in there that
I can't read them and so can't read the differences.

Any other approaches?

Cheers
Mark