The Great Divorce is my favorite book of his. I just finished my third (or fourth) reading and I have liked it better with each reading. If you are unfamiliar with the story, Lewis wrote an allegorical story of ghosts taking a bus trip from a perpetually grey city to a beautiful land.
Each of the ghosts who make the trip share the common feature of self-centeredness, though it manifests in different ways. They encounter bright Spirits who live in perpetual joy in the presence of the Trinity. Describing one of the beings at the end of the book, Lewis wrote "The Happy Trinity is her home; nothing can trouble her joy."
I think that in addition to being a well-written, deeply engaging story, another reason I am drawn to The Great Divorce is that it seems to present a fictional account of what may become in a community centered on God rather than on self. As a student of Larry Crabb's work, The Great Divorce reminds me in ways of Shattered Dreams, which interestingly relies significantly upon Lewis's idea that if you put first things first, you will get second things thrown in, but if you put second things first, you lose both first and second things.
I cannot recommend this book highly enough.