I recently read an article about Booktype, got interested and tried software myself.
Booktype is developed by Sourcefabric, a non profit organization that provides couple of other interesting solutions like Airtime (radio automation), Newscoop (content management system) and Superdesk which combines Airtime with Newscoop as a suite for independent media.
Booktype uses Django web framework. Software is easy to install but you can’t forget about Objavi 2 (FLOSS Manuals book exporter) to save your work in any of the available format.
Interface is simple and readable built with collaboration in mind. Each book has its own history, custom versions and permissions. Books are written in HTML using TinyMCE editor and can be exported to PDF, ODT, EPUB and MOBI format.