How can I make personal documents on my external SD card visible to the Kindle app?

I have a tablet running Android 9, with the latest version of the Kindle app. (The tablet is a Galaxy Tab A (8.0", 2019), but that shouldn't matter.) It's not rooted. In the Amazon Kindle app, I've enabled "Download to SD Card" (which causes books I've bought from Amazon to be stored on the external SD card) and "Show Personal Documents in Library". The latter option makes *.mobi files I've copied into the "Books" folder in "Internal Storage" to be visible in my Kindle library. The path to the folder is /storage/emulated/0/Books. The external SD card is visible as /storage/0000-0000. Copying *.mobi files into /storage/0000-0000/Books doesn't make them visible to the Kindle app. Is there a way to copy my own *.mobi file (not purchased from Amazon) to a folder on my external SD card so I can read them with the Kindle app? I'm guessing there's a particular directory path, but a Google search has not revealed what it is. (I know I can install books by emailing them to myname_blah@kindle.com, but I'm looking for a way to just copy the files to the device.)