My college admissions advisor was reading these books when I looked at my school and recommended them. On a fit of C.S. Lewis longing, I picked them up off the shelf and my whole world view changed. Set on Mars, Venus, and Earth, these books explore all kinds of ideas about the Fall, redemption, and beings beyond our comprehension (we call them angels and demons, here they are the eldila).
This book made me really think-- especially after reading bits of Paradise Lost, Dante's Inferno, and taking Developing a Christian Mind. I had to stop reading and just stare out to window (I read much of the series in a car) and contemplate. Each planet, according to the book, are ruled by an Oyarsa, an archangel that speaks for the planet in heavenly courts. The Oyarsa for Earth (called Thulcandra) is bent. He has led the world astray and carefully turned it from God to a destructive path.
The idea of being bent, not broken, really intrigued me. It's true that Satan has no use for a broken man. A broken man serves as a warning to all the men on the straight path as to what happens when one leaves the Lord. A bent man, though, the one who is a rich, handsome, successful man who does't really make to church on Sunday but says he's a Christian but he doesn't really make time for his family.... you get the picture. If Satan can just bend our wills away from God, we can easily serve him while appearing to be a Christian, living for God.
Other very deep ideas pervaded the books and I will certainly be reading them again, but this one stuck out to me the most.
These books have a fantastic story, classic Lewis pacing, and deep deep thoughts behind it. Read it and travel beyond your imagination.