- Outline the book so the work can be broken up and worked on in individual pieces.
- Use Google Docs with a shared folder (can be shared with specific Google users by user id)
Those two things should get you there because Google Docs will even allow multiple users to edit a document at the same time and show the live edits (in real-time) which are done by each user.
It will also allow users to read / edit any content in the directory so each writer can work at her own pace and examine any of the documents.
You can also turn on revision watch so you can see who made revisions to the document and notes can be added by the revising user.