Please update to last BookLibrary Pro version(ordasoft.com/book-library-joomla-ebook-extension
But update to free version also must resolve this error.
Before making any upgrades, make backup yours site( files and DataBase). With that you will not lost something important
Please do full XML export.
At first please check upgrade/update process at the test website!
If you have custom changes of component or site, you can lose your changes.
Please, in that case do not make update.
If you would like any further information, please do not hesitate to contact us too.
all below - please first do on test site:
1. do backup below folders from your site
Best if you have full backup(files and database) for site.
2. on your site - please do full XML export of the books
3. install to it new version bookLibrary
Check how all work