 I think of it as the Ransom trilogy because of the hero Ransom appearing in all three. So let's call it the Ransom trilogy. So the middle book, and you say it was Lewis's favorite book, and boy, he wrote a lot of books. Yes, he did. It's called Perilander. So it's a novel? Yes, a fantasy. He would have called it a romance, using the old medieval classical term for an adventure.