Audible audiobooks gone after updating to Monterey

I recently updated my MacBook to Monterey. Previously, my audiobooks had been stored on the iTunes platform. These audiobooks included audible downloads that I no longer have access to through my audible account (since I have closed it). My iTunes library is stored on an external hard drive. I made a time machine backup before I updated to Monterey. After updating to Monterey, all of my audiobooks have disappeared. They are not available in iTunes and they do not appear in the Books app (none of my previous audiobook library is available there). Have my audiobooks been deleted? Do I need to revert to my previous OS to access them? Are they still in my external iTunes library but inaccessible? Can they be migrated to the Books app without reverting to a previous OS?