This module enhances the book module. It introduces a permission "add child page to any book". For users with this permission, the book module will behave as usual. Users who are not granted this permission will only be able to move a book page within the current book page's hierarchy. All other hierarchies will be hidden. Furthermore, these users will only be able to add a new book page through the "Add child page" link on an existing book page.
This module is not under particularly heavy development but is probably not ready for your production use. Please test it under your circumstances before deploying.