Though not as well known as Lewis's Narnia novels, he also wrote a series of three novels, featuring Elwin Ransom as the main protagonist, in the late 1930s and early to mid 1940s. Lewis wrote the novels due to his famous conversation with his close friend J. R. R. Tolkien, who both said there wasn't enough of their type of fiction in the world, so they would have to write their own. Lewis wrote the Space Trilogy, and Tolkien wrote part of an abandoned, unfinished novel called THE LOST ROAD. Ransom, a philologist, is actually modeled after Tolkien.
There's a story in one of Tolkien's letters (published in LETTERS OF J.R.R. TOLKIEN) where his daughter, Priscilla, was reading the trilogy during one of the holidays in the 1940s, and, according to Tolkien, quite sensibly came to the conclusion that PERELANDRA was the best of the trilogy.
The second book in the trilogy is PERELANDRA. In many ways, it is the richest of the trilogy in terms of spiritual depth. While OSP is more straight science fiction, and THAT HIDEOUS STRENGTH social commentary, PERELANDRA is closely modeled on the Adam & Eve story in Genesis. This novel is something of a homage to one of Lewis's favorite, John Milton. It's a beautiful book, and raises the question of what exactly would happen if Adam and Eve had not fallen. And this time, instead of being kidnapped Maleldil sends Ransom there.
The main plot of the novel has Ransom, who was kidnapped last novel, actually being sent to Venus. Lewis does away with the problem of spaceships this time around, having angels just take his protagonist there. He finds himself in a world of vast oceans, with floating islands that are actually behave like film or foam on the ocean, undulating and taking the shape of each wave. He soon meets the Green Lady, who is unfallen. Her husband is on another of the floating islands (they had become separated when they were on different islands which drifted away from one another).
Eventually, Weston arrives, the villain from the previous novel, and we find that Ransom must prevent him from corrupting the Green Lady, to prevent another Fall into Sin. Weston is an agent of Satan, and so wants to bring sin into Venus as well.
The majority of the novel focuses on Ransom and his efforts to protect the Green Lady from the Un-man, which Weston actually becomes after shortly arriving on Venus. Weston actually becomes demonically possessed, and ultimately must be stopped at all cost. Ransom is stripped, both physically and symbolically, having to rely on Maleldil (Jesus) to help him.
Eventually, Ransom and the Un-man swim to an underground chamber, with the Un-Man biting Ransom's heel. This wound that will never fully heal, an allusion to Arthur and the Grail myth as well as the scripture in Genesis saying man will crush the serpent's head, and the serpent will bruise mankind's heel. In the end, Ransom puts the Un-Man to death, and so prevents Venus from having a second fall. The Green Lady and her husband are united.
The descriptions of the floating islands and Ransom's experience on Perelandra in the first section of the book before he meets The Green Lady, along with the ending section of THE LAST BATTLE from Narnia where they are in heaven, to me is the most beautiful passages that ever came from Lewis's pen.
One fault that this novel does have it the ending seems to be rather preachy, but otherwise this is a first class novel, and for many readers this will be one of Lewis's most spiritually rewarding novels. Only in THE SCREWTAPE LETTERS does he deal so accurately and directly and with such psychologically insight on the problems of temptation and accountability.
While this novel is technically termed "science fiction", this is much more a spiritual track of our times than straight science fiction. The book is closely modeled upon Milton's PARADISE LOST.
For myself, the best way to read this book is reading it in conjunction with two other books, an unofficial trilogy, if you will. Because PERELANDRA is so closely related to PARADISE LOST, you should read that as well. Also read Lewis's literary criticism A PREFACE TO PARADISE LOST, in which he expertly discusses Milton's work. Lewis is a brilliant literary critic, and PREFACE is one of the best critical works on the PARADISE LOST ever produced.
My own unofficial trilogy:
1. Paradise Lost. (John Miltion)
2. A Preface to Paradise Lost (C. S. Lewis)
Overall, many readers will find PERELANDRA Lewis's most spiritually satisfying of the three novels. The characterization is strongest in this novel, as Lewis is only dealing primarily with three characters, and we really get to know all of them quite intimately. The novel is also focused mainly on evil in an unfallen world and what one must do to save that world. Until Lewis wrote TILL WE HAVE FACES in the late 1950s, the novel which he felt was his true masterpiece, he long felt this book was his best, and placed it second best after FACES. This was Lewis's favorite in the Space Trilogy and for good reason. It's probably the best (though my personal favorite is THAT HIDEOUS STRENGTH).