What is a book?
A book is a piece of content that can have child pages added to it.
Automatic links are added to the pages, giving forward/next and up links that easily move you through the book. All child pages are automatically linked to the parent page.
This Drupal guide is set up using a book. You can see the automatic links that are added on the front page (shows all the sections of the guide) and the back/next links below on every child page.