This movie was based on a short story by the brother of the director. Or at least, it was based on the idea of that short story -- it wasn't actually written at the time Christopher Nolan heard about it. But it was intriguing enough for him to get his brother's permission and start working on the screenplay in fairly short order.
That short story, titled "Memento Mori," is included on the DVD of the movie Memento. Its main character is not named the same as the movie's, but it's essentially the same guy and the story forms a prequel of sorts. Once you've seen the film, you can read the story and find out a few things about the main character that the movie doesn't discuss, and his motivations make much more sense.
Sitting between these two media is a Web site at http://www.otnemem.com which is also included in its entirety on the DVD. It doesn't discuss the movie much, but collects a series of photographs, news clippings, notes and writings directly from the movie and from the events around it. They partially bridge the gap between the short story and the film, and fill in Leonard's motivations even further.
It's an interesting way to use multiple media to tell a complete story, really -- literature, film, and multimedia each telling part of the story's whole. It's also a very effective use of the DVD media to give the interested viewer something besides the usual deleted scenes and behind-the-camera tripe. Of course, you really ought to watch the movie first. The other two would just spoil it all for you.