I’ve downloaded a couple of personal books and saved them to iTunes on my Mac. They’re AAC files created by AudiobookBuilder. They show up in iTunes in the Audiobooks library on my Mac, but for the life of me I cannot find how to import them into the Audiobooks collection of Books on my iPhone. I have Mojave on my Mac and iOS 12 on my iPhone. iTunes on my Mac can’t even find my iPhone to sync with it. Aaaarrgh! Help!
Update: I noticed there was an iOS update today. Once I downloaded it, everything is working fine, iTunes on my Mac found my iPhone, and my audiobooks are now synced. What a pain though.