No doubt lots of great books to purchase, but don't forget that there are a vast number of both free or inexpensive books to download. See here for more info on free classics for example:
One other cool feature is the custom e-mail feature. You'll get a personalized kindle.com e-mail address which you can use to send PDF's, Word documents, and even JPEG's or GIF's directly to your Kindle.
I've also signed up for free 14-day trials for a number of big newspapers, such as the New York Times or the National Post. Every morning when you turn on your Kindle's WiFi, Amazon's Whispernet will immediately update your Kindle with new newspaper subscriptions. It is so nice to have the world's newspapers at your fingertips every morning.
Finally, my first purchase and download was my father's latest book "Kruger's Gold". I've read his book before of course, but am enjoying it all over again via my Kindle. See here for more info on his book.
Look for more postings here in near future as I learn more about my Kindle.