While I thoroughly enjoy Lewis' space trilogy, this is the book I have the most trouble with. I simply could not make many of the connections between Perelandra and what Lewis was saying about God. I don't know if this is because the book is more dense than the others, if I read it at a bad time, or if he relies too heavily on Anglican ideas (and other European Christian ideas) that are distant to my world view. I did get the main points. I also would not discourage a person from reading it, it was worth the challenge. I am keeping my copy in the hopes I can better understand it at a later date.
At any rate, this is the middle book of Lewis' space trilogy. Ransom must go to another planet to stop evil from setting up its carny show among the young fresh new royalty of the young fresh new planet. In other words, Ransom is put into place to stop the devil from ruining this other Garden of Eden. I do recommend the book. I'm going to have to go read it again.