There is no way of organising books on the device itself - except you can choose to have them displayed alphabetacally per author or title, or latest opened. We are all waiting for Amazon to add tags or some other functionality to organise content.
If your books are DRM-free, you can use a library application on your computer to manage your books. A great one is calibre - and it's free, too:
http://calibre-ebook.com/
In calibre you can add categories to your books, and convert them and manage the content on your Kindle as well.