This book is the first in the Space Trilogy by C.S.Lewis. It is set on Mars. It is an allegory, and not everything that is said should be taken as truth, not even the spiritual stuff. Essentially, Earth is a blockaded planet, cut off at the moon's orbit, due to sin. There's an angel that rules each planet, but ours is Satan. It's the beginning of the war to take back Earth from Satan and co. Very good.