How to get Kindle to recognize files as "books," not "docs"?

[anolinde.livejournal.com]

Hi! So, I recently got a Kindle Fire and am attempting to put ebooks (including fan fic) on it. The only thing is, even though I drag the files into the "Books" folder on the computer, they only appear under the "Docs" tab on the Kindle, and I'd like them to appear under the "Books" tab. Is there a specific file format you're supposed to use that I'm just not realizing? I've tried .pdf and .mobi, and neither of them have worked. I've heard that .azw is a Kindle file type; should I try converting to that? And if so, how?

Thank you for your time!