Steps to recreate de bug :
1- Enter a course and create a new Book resource
2- Add a simple structure in the book
Chapter 1 - ...
Sub-Chapter - 1.1 - ...
Chapter 2 - ...
3- Make a backup of the course
4- Restore the backup in the same course. Overright the current one.
5- The book still exists but if you click in it there's no content.

Book module version 2.0Dev is installed and all functionnalities work fine except for this one.