he's not a tame lion
Occasionally I run into to people who haven't read The Chronicles of Narnia, and I'm always surprised. The books (The Lion, The Witch, and The Wardrobe, and its sequels) are juveniles, but well worth it nevertheless. I anticipate reading them aloud to our child(ren). (Our first arrived on February 12, 1997.)
Lewis also wrote three books sometimes known as the Space Trilogy: Out of the Silent Planet, Perelandra, and That Hideous Strength. When I first read these in my early teens, I liked the first one quite a lot, found the second one tough going, and couldn't finish the third. Now I find the first one kind of trivial, the second more interesting, and the third quite good. Like most of Lewis' work they are written from an explicitly Christian viewpoint, which might be offputting to some, but is a source of interest to me.
The Screwtape Letters is a series of letters from Screwtape, a senior devil, to his nephew Wormwood, an apprentice tempter. Wormwood is trying to tempt a human being into Hell, and Screwtape is advising him on the best procedures. While written from a Christian viewpoint, it may be of interest to non-Christians for its sterling depiction of how people can be nasty to each other in little ways. It is quite illuminating.
The Chronicles of Narnia
I've noticed that the editions of these books currently on U.S. shelves have them ordered by series chronology, rather than the order in which Lewis wrote them. I think that's a mistake, personally, so the order below is the traditional order.
The Space Trilogy