I'm not just a hipster curmudgeon, hating on everyone's favorite author for the excitement of it.
Lewis is an excellent writer. But like any writer, he has strengths and of course stronger efforts.
This trilogy, the second installment especially, shows Lewis at, or slightly outside of, the limits of his strengths. Those strengths are prodigious, sure. And it's ok to be good-great-at nine genres, and not have science fiction not be one of them.
One of those books that, about fifteen percent of the way in, you realize these uncomfortable characters and this oddly limited setting and this plot we're going to be the whole book. And because it's so well written you know you'll enjoy it, but because the author has Something to Say and because he's limited what will happen to a small set piece, you will spend the whole book feeling robbed of some great ignored potential.