This is one of the best books I've read in years, probably the best since The Napoleon of Notting Hill in 2006. In this sequel to Out of the Silent Planet, some time after Dr. Ransom's accidental visit to Malecandra (Mars), the eldila visit him and send him to Perelandra (Venus) on a mission that will determine that world's future. The book is full of spiritual and theological thought, and the end is mostly poetry sung to God.
What my Goodreads friend Laura K says about Perelandra is fair. The science-fiction elements of this book are significantly less interesting and developed than those in Out of the Silent Planet. Basically, it's mostly theological in content, and doesn't try very hard to be science fiction. But I can't say I found that objectionable. After reading its first two books, I love the Space Trilogy. Clearly, fewer people read it than the Narnia series; their loss.