Certainly not an easy book. Even adults, if they prefer their Lewis fiction to be more uplifting, may have a hard time with a book that opens with "I am old now and have not much to fear from the anger of gods". But the payoff is so worth it. I'm glad you ended up liking the book.
I note that it's not an easy book for the young. I bounced off it, hard, in my early teens, when I read everything else of Lewis's writings.
Certainly not an easy book. Even adults, if they prefer their Lewis fiction to be more uplifting, may have a hard time with a book that opens with "I am old now and have not much to fear from the anger of gods". But the payoff is so worth it. I'm glad you ended up liking the book.