Everything I've found online lists them as being located in:
Android\data\com.amazon.kindle\files\Books
There's a few books in there, but the book I just purchased and am currently reading is not in there.
The articles referencing the above directory are all old. Have they changed the storage location? Are there any other places they could be?
DEVICE: Amazon Kindle Fire HDX 7 (3rd Gen) - Model C9R6QM
RUNNING: Fire OS 4.5.5.3 (June 2019)