An amazing book.
I was a little concerned about how I'd like this book, since I was kind of ambivalent about Out of the Silent Planet. Rarely do you see a sequel that is better than the first in the series. This sequel, however, was not like the difference between "The Empire Strikes Back" and "A New Hope" - it was more like the improvement of "The Empire Strikes Back" over any of the Star Wars prequels!
The description of Perelandra has you guessing for a while - until you realize that what he is describing is from first impressions to dawning comprehension of reality. The novelty of the lands of Perelandra (if they can truly be called that) are enough to write a whole sci-fi book about (much like how in Ringworld the planet's setting is the central focus of the story). However, for Lewis, this is merely an imaginative setting - he's got bigger fish to fry.
And what a fish it is! The story of the Garden of Eden, retold with a fallen human being there to try to avert it! Lewis's dialogue of the temptations thrown at the "Eve" of Perelandra are so cunning and so intricate that there is a real tension of not just "how will they get out of this one?" but "is it even possible to get out of this one?" Truly an amazing job that puts that whole serpent and apple business to shame.
Highly recommended for anyone who loves sci-fi and is serious about their Christian faith. Even if you are not a devout Christian, you will be intrgiued by what is presented, philosophically and fantastically, in this book.