Fancy meeting you here.... I am a huge fan of C.S. Lewis and have read many of his works, but not all. I can highly recommend The Chronicles of Narnia, The Screwtape Letters, and The Great Divorce to new readers of Lewis. Once you have read those you can move on to some of his more serious works, Mere Christianity, A Grief Observed, Till We Have Faces, and many, many others.
Also, for anyone who is interested, there is an interesting group sponsoring a national "tour" to stimulate interest in C.S. Lewis and Narnia as a buildup to the movie. I attended the Minneapolis event last Friday and found it very interesting. I learned a lot about Lewis that I didn't know before. Check the Narnia on Tour website below for more info about upcoming events in your area:
Just checked out those Narnia on Tour dates. Turns out one is at the Borders Books in our town, and another is at a Borders books not too far away! We have our choice; cool!