Out of the Silent Planet
This is the only Lewis I've read aside from the Narnia series. Being as I was raised dirty heathen, I didn't pick up on the Christian aspects of his writing until later, and that still isn't the first thing that springs into view when I read him. Just so you know.
I think my favorite parts of this one are the descriptions of Ransom lying in the spaceship, watching space go by. For someone who'd not only obviously never been to space, but didn't even have any descriptions from others who had, Lewis paints a detailed and engrossing picture of the teeming heavens. The whole book is obviously influenced by HG Wells' First Men in the Moon, but not enough to keep it from being very much its own novel.
Now, don't get me wrong - there's a fairly simplistic "man is often evil due to the presence of Satan/bent Oyarsa on Earth, but the peoples of Mars are good, kind, and wise because they have true angels/true contact with God" theme running through the novel. Being as it's Lewis, I guess that's par for the course, and it didn't keep me from enjoying the story itself. I was intrigued by the idea of the various Martian races seeing each other as both human and animal (and thereby not needing pets in the way that Earthlings seem to, as a connection to the animal world within our own culture). I suspect that some folks right here see other races/nationalities the same way, and not in the respectful way Lewis lays on his creations.
You know, I think this book (or the whole series - I haven't got far enough into it to know) helped inspired L'Engle when she wrote A Wrinkle in Time. Ransom's discussions with the different races is echoed in some of the childrens' encounters as they travel outward from Earth. Plus, there's the image of our planet being shrouded or silent - set apart from the rest of creation.
final thought: It really all comes down to whether your believe that "Our right to supersede you is the right of the higher over the lower." I don't, and I enjoy the case made by Lewis on the matter as much as I enjoy his descriptions of the petrified Martian forests and the bright, warm stretches of space.