If a user is reading a book and want to share it, he is going to copy-paste the current URL he is reading.
But this URL will not work for users who are not reading this book. So the idea is:
When somebody goes to read-book url, if 500 error is coming, redirect to book url
OlegKlimenko
commented
6 years ago 
If a user is reading a book and want to share it, he is going to copy-paste the current URL he is reading. But this URL will not work for users who are not reading this book. So the idea is: When somebody goes to read-book url, if 500 error is coming, redirect to book url