Can more than one person work on a book at the same time?

It depends on what you mean. 

Yes: A book can have more than one administrator. A book can actually have a limitless number of administrators, authors, subscribers, editors, and contributors as long as those users are eligible to create an account on your Pressbooks network. 

For more information, check out our chapter on Users & Collaborators.

No: A chapter cannot be edited by more than one user at a time. Pressbooks inherits the default post edit settings from WordPress. This system does not allow for collaborative live edits. Only one individual on one device can make changes to a post at a time.

If another user tries to access that post, they'll receive a pop up that lets them know that another user is currently editing the post. They'll be given the option to "Take Over" the post, preview the chapter, or return to the previous screen. 

If they choose to take over the post, any changes the original user has made will be saved as a revision, and they'll be notified that the post has been taken over by another user. 

This only applies to individual posts, including front matter, back matter, chapters, and parts. Many users can work on the same book at the same time without issue, as long as they're not editing the same specific posts.