ok - so it is really a moon+ issue, but not one that is ever likly to get sorted.
to leave with a final example. if i choose to to read the book title1 via CC, moon+ creates a database entry for storage/emulated/0/books/title1- which is the path that CC passes to it
but if I locate the same book from within moon+ , via my files - it creates ANOTHER database entry for /sdcard/books/title1
both entries point to the same file but each entry has it's own saved record of last page read....
i dread to think how confusing d get it the mutli user option was also in play on the android device
with an actual SD card in use as the books folder though, both programs do agree on what the path should be, and deleting a finished book from in CC removes the recent entry in moon...
google tells me that
sdcard is a symlink, I am not an android expert.
if I follow your explanation though, because this is an android 7 tablet, my books are being stored twice, as moon+ has to create its own copy. I am pretty suremoon is not creating a copy, because zapping a book from in CC, causes it to become file not found in moon. Maybe android 8 is the release that's going to change that