A few years ago I co-authored a book. Collaborating on the table of contents is a critical part writing a book with a parnter. After several futile attempts to exchange and revise Word outlines, my co-author and I ended up going to a cabin in the mountains, and working around the clock to hash out the outline.
Had it been available then, Thinklinkr is a free online outliner that we could have used instead. Highly recommended.