It is another way to convert text into e-format.
But I suspect that the problem you are having as all of us are, that the kfhd will recognize some books as books and most others that are non-amazon as documents. I am sure that this problem has users pulling their hair out because books and such non-amazon content ends up in the document folder instead of the books folder and are displayed as such. It is sorta the same with audio books, if they are not bought from amazon or in the audible format. They are not shown or playable under the audible player and are only playable as music files.
It seems that if the books or documents are not connected with amazon in some way or fashion they will not be shown under the fires menu as they should or wanted.
It sorta sucks that the fires main displays menus are only for amazon stuff.
The program I linked (have not tried with the fire) does a good job of converting other formats of text into kindle or other reader e-format. It is just another tool besides calibre to use.
Tho calibre IS the Swiss army knife of converting tools available.