Amazon's instructions for sideloading to a Fire tablet are
here.
That states when connected via USB the folder to copy to is named "Internal storage/Books". Books must be placed into that exact folder. You cannot use subfolders. I don't know what internal folder name this corresponds to.
Also, to show up a book must be marked as a book (cdetype=EBOK) in its internal metadata. If not then it will be treated as a personal document. Those do not show up in the "Books" app. You need to use the separate "Docs" app to view them.