Unable to edit author or book art

Unknown Author and missing cover art. Using homemade m4b files, with the info updated. Title, Authors, and album art are all set. The thumb shows in Finder, and I see the details under <more info>. When I import these files into Books the results are inconsistent. The cover art sometimes disappears, and maybe the author comes up Unknown. OK, I get it, but how can I correct this? I don't see an edit Audiobooks option anywhere in the Books app. What simple checkbox am I missing? How can I edit the details for the audiobook?