Audio Books

I download audiobooks via my local library. These come from a site called "Bolinda Audio Books". I unzip the files successfully in my Windows 8 PC. I then add these books to iTunes, successfully. I connect my iPhone 8 to this authorized PC. It goes through the backup / Sync, successfully. I then try to "Click & Drag" each book into the iPhone on the side panel of iTunes.......... Nothing, nada, zip! They do not transfer. I have been using this way to install audio books and music for years. Haven't needed to do it for some time. And now it simply does not work. HHHHEEEELLLLPPPP! Please.