The documents section can be configured so that some sections are only available to certain people.
To change the access permissions for a specific book, navigate to the main book page. Then, click on the "Book access" tab.
This page allows you to set - based on the role - who can view, edit, and add things to the book. You can also allow specific roles to administer these access control for this book alone if they don't have the Document Admin role (which allows them access to the administration pages of all books).
Note: Site Admins and Document Admins have the ability to see and edit all documents/books.