You wouldn't be able to integrate it easily with Booktype's front end interface. What would be possible probably is taking output from Etherpad and feeding our publishing engine with that content with PDF or EPUB output. Not sure how to organise Table of contents and chapters, headings, subheadings and other things. Maybe some other editor with concurrent editing feature would be more useful.
Not sure how Etherpad internally handles things to tell you more but the only way is having brand new front end interface which works with Etherpad (for instance, each chapter is new document) and which is capable of feeding our publishing engine.
Thanks for your answer.An example ofthisintegrationis implemented inthis project:http://escritura.proyectolatin.org/ It is a veryinteresting example ofreal-time collaboration