Amazon keeps changing the rules. It is as if they don't want third party books to show up.
The best I have seen done is that the books appear on the "lastest used" page.
Amazon stores books in the Android data folder, IIRC (my fire is packed away) Android/data/com.amazon.kindle. I think they break the books apart into separate folders, one for annotations, one for text, and one for who-knows-what.