Safely delete book and its child pages
book_delete module will address your issue !


  Book Delete adds the ability to delete entire books to the book
  administration page (admin/content/book). It uses a batch operation to
  allow deletion of large books.


Take a backup of database before doing any delete actions :-)

It just worked as expected !! Follow below steps.


Install module
Clear Drupal Cache 
Go to admin/content/book
Select the book you want to delete and click on delete book link next to it.
Confirm delete !


That's it :-)