Because you can add books in Safe Mode we know it is
not a corrupt install, configuration (preference settings, plugins etc), or library.
The most likely candidate is "Norton's AV" - I found this
How to Exclude Files and Folders From Norton Antivirus at the top of a search for 'norton exclusions' - followed by more than a few more.
The three folders you want to exclude are described in this post ==>>
https://www.mobileread.com/forums/sh...62&postcount=3. I would exclude them one at a time - Library, Configuration and Software.
If excluding the folders from Norton's doesn't resolve the issue, then it must be something you have in Start Up. Remove the bottom 50%, reboot and test. If the problem goes away the culprit is one of those you removed, otherwise its one of those remaining. Then repeat the process on the appropriate 50%, rinse and repeat, until you isolate the culprit. Then make a decision as to what to do about it.
BR