You may have owned a collection of ebooks in various, non-DRMd formats before getting your hands on the Kindle Fire, and you may have transferred (or, “sideloaded”) these books onto your device. And if you did, you may have been surprised to find your collection listed under “Docs” instead of on the “Books” page of your Fire.
CNET has an excellent how-to article that explains how to use Calbire, a free ebook management and conversion tool, to get your existing ebooks onto the Fire in such a way that they will show up on the Books page. From the article:
The benefit of having your e-books listed under “Books” is that you can sort them by author. Under “Docs,” you can only sort by recent or title. If your collection is a modest one, this might not bother you a whole lot. If, however, your collection is rather sizable, you might want the extra sorting option…
…A side benefit to converting your e-books in Calibre, is that it shrinks the size of the Mobipocket file (.mobi). Since the Kindle Fire doesn’t have any memory expansion slots, converting your e-books may end up saving you a lot of storage space on your Fire.
Read the full, illustrated how-to on CNET.