|Folders on my Kindle Fire|
May not be the same on yours...
On your Kindle Keyboard, and Kindle Touch, the folder structure is a little different from the Kindle Fire. The Documents folder is where your books are, and any pdf or Kindle compatible file dragged into this folder will show up on your main (Home) screen as a "book" or document for you to read.
The Kindle Fire is a different animal. It has a folder (Books) for things you have purchased from Amazon. I suppose you could drag Kindle files (.azw) into this folder too and they are treated as "books," meaning you touch the word "Books" at the top of your Kindle home screen and anything on your little machine will be listed, with a nice link to the "cloud/archives" too. There is another link to "Docs" at the top of your Kindle Fire main screen too... This is the one that will take you to any personal files (like pdf and other files) you may have stored on your Kindle.
When you look at the folders on your Kindle Fire the next time you connect it to your computer, be sure to notice the separate Documents and Books folders. Now you know why.