Hi there,

You can definitely co-author books using BookSmart. Many users have done so. You will need to pass the book file back and forth each time, to see each other’s changes. The (only) way to do that is via File>Export Book, as shown here.
http://blog.blurb.com/index.php/2007/05/17/how-to-archive-share-or-transfer-your-book-files/

Regards,
Michael/Blurb Customer Support