You can save setting in Book Library - often that happened when you manual change permission for files and folder inner yours Joomla installation.
So now yours HTTPD server hasn't permission create/edit files in folder:
You may with help FTP or cPanel - check permissions for that folder and set it to: 777
Also please check permissions for all files in that folder and set its as: 666
also you can reinstall Book Library.
If that not help - please ask yours host provider.