Brainstorming, writing, citing, editing - these are all phases in writing a book or article, and co-authoring can be done with traditional tools such as word processing and dedicated face to face contact. If you want to write with others, but are time-poor, tradtional tools can be limiting. Enter cloud-based tools that allow each writer to contribute when they can, where they can.

Courtney Greene and Elizabeth Ruane wrote about their experiences in Collaboration in the cloud: Untethered technologies for scholarly pursuits. They discuss the process and a number of different tools they used. You can see more tools here.