There is a option to remove the any page from the Book outline. Suppose I have created a basic page and put in a Book outline. Now if I want to remove, click outline as shown in the follwoing image
Now, you can see
Remove from book outline in red in as show in following option. That will remove that Basic page from the book.
You can not delete entire book at a once. Why, because Book is just a indexing tool, It is not a content type. Book page is a content type. Book page and Book are two different thing. And it should be that way. Hence, you can delete all nodes of one content type by deleting that content type itself. But, if you are putting some node under book outline, you are not creating a entire group/content type, You are just indexing your node in specific order.
Hence, If you delete any Parent item in book, It shows a message that the
child will be re-ordered automatically. Hence, you can understand its just a ordering/indexing tool.
Hence, to provide this feature Book Delete module was created. But still it does not have Drupal 8 Version.