Whenever you open a book in Contribute, you must put the book into Edit modebefore you can arrange its contents.
When the book is in Edit mode you can rearrange, add, or remove topics and objects such as the Table of Contents, the index, and so forth.
When working in Edit mode, the contents of the book are locked for other users. Other users can open the book as read only, but are unable to rearrange the book or add content.
Unlocking the book takes it out of Edit mode and makes it available for other users again.