I looked at the folder the download from Amazon added to my Kindle for the book. The largest file in there (which I assume is the actual text of the book) is an AZW6 file (not AZW3). Using AZW6 for a Search here, I managed to find the thread I reference earlier:
https://www.mobileread.com/forums/sho...highlight=azw6
The response there was that AZW6 files are not supported in Calibre (and to try re-downloading the book from Manage Your Content and Devices). Since I only seem to be able to get an AZW6, am I SOL?