Solved!
With Moon+Reader Pro 4.1.2 and Android 6.0.1, go to My Files in Moon+Reader and select Local Files. Navigate to the root directory (/), then to Storage, and then to [name of your external SD card -- mine is 3209-EF9D]. Then you can go to wherever you put your ebook library.
This was suggested by Purple Lady's 4/21/2013 reply to a similar question. With the Moon+Reader and Android versions that were current then, the path was Local Files > root > mnt > ExtSDcard. That's different from the path for the current versions, but it was enough of a clue for me to find my books. Thanks, Purple Lady!!
