I was sorely disappointed by Lewis's Space Trilogy. The entire trilogy could be summed up as follows: "And God smote his enemies." Three books building up to a final conflict, and in the end it feels like one side wasn't even needed, since God takes care of it anyway.

To boot, his use of allegory on Perelandra was just hideous. If you're going to make an allegory, you don't have a character explain the allegory while it's in progress!