First I have to admit that I have not read much CS Lewis. Given his reputation as one of the premiere Christian writers of the last century I recently purchased a small stack of his signature series books. The Great Divorce is the first one I have read, and I was truly amazed by its simple, yet complex message.
This book is powerful fiction with enough theology to put the gears of your mind into overdrive. In my opinion Lewis addresses two key questions in The Great Divorce - Is there a difference between heaven and hell? And, does God truly give us the freewill to decide our own eternal destination? Lewis really makes the reader ponder these two "deep" topics, and think about why certain "types" of people may have more difficulty choosing heaven over hell.
I won't give away Lewis' conclusions, but instead I will highly recommend this book to anyone looking for a short, but powerful book. If you are a Christian you should definitely add this to your reading list. Lastly, if you are not a Christian, don't be afraid to read it. Lewis was a great writer by anyone's standards, and his "theology" is subtle enough to be enjoyed by anyone.