As we see in the Chronicles, there is Aslan's Country which represents Heaven. There is a tease of Aslan's Country in Voyage of the Dawn Treader. Reepicheep goes there in a coracle, and we don't see what happens to him afterwards. Two men, Enoch and Elijah, both went to Heaven without dying. Reepicheep going to the Aslan's country in a coracle can almost be viewed as Elijah going into Heaven in the fiery chariot.
As Caspian says in The Silver Chair after Aslan resurrects him to His Country-
“I see what’s bothering you. You think I’m a ghost, or some nonsense. But don’t you see? I would be that if I appeared in Narnia now: because I don’t belong there any more. But one can’t be a ghost in one’s own country. I might be a ghost if I got into your world. I don’t know. But I suppose it isn’t yours either, now you’re here.”
He was saying that he would be a ghost in Narnia because he was not living there anymore. He knew now that he was in Aslan's Country.
The after life is very complex topic. In The Last Battle, when Tirian puts him and Rishda into the Stable, it can almost be seen as they were crossing over to the eternal horizon. Tash shows up to claim Rishda as his "lawful prey." That is not saying it was because Rishda was a rich and powerful man. You might recall a story that Jesus told about a rich man and a beggar who both died; the beggar went to Paradise or Heaven and the rich man went to Hell. I know that Hell is not a very comfortable topic; even some pastors leave it out in their sermons. But since the Bible teaches it, it should be taught. The rich man didn't go to Hell because he was a rich man; it may have been pride and selfishness and cruelty. Rishda being Tash's lawful prey not because he was a Tarkaan or a rich and powerful man could be the same idea as well. Could it have been selfishness and pride and cruelty as well?
As Narnia is coming to an end, every creature in Narnia looks at Aslan. Then they turned either left or right. The decision that each character in Narnia made in life determined where they've spent eternity; the decision we make in this life will also determined where we'll spend eternity.
The description of Aslan's Country is beyond comprehension; the description of Heaven is beyond comprehension as well. You can imagine when John had his vision of Heaven, he had a hard time to take it all in and even described it when he wrote Revelation. The after life is way beyond than what we can imagine.
The After life is always a difficult subject. I feel C.S. Lewis treated it pretty good. I watched a show recently (not at all Christian or even "spiritual") where several seconds before that character died she said, "I'm coming (dead friend's name inserts here)" My point is that many fictions try to address the subject and I feel C.S. Lewis with his Christian conversion had an advantage some others don't. I like that in Magician's nephew Diggory rejects the eternal earthly life by saying, "I don't want to live on when everyone I love and know is gone I would rather die and go to heaven." (not sure that is the exact quote but it is a close paraphrase)
He does all things well.
The after life is very hard to grasp.
I think it's also great that CS Lewis focused more on the Heaven of Narnia and not Hell. He doesn't tell us what becomes of Rishda after Peter banishes Tash in the Name of Aslan. He doesn't even tell us what becomes of those who turned left as the world of Narnia is coming to an end. All we get is that they disappeared in the shadows, never to be seen again.
It was all the matter of what choice each character made in life. That determined where they've spent eternity. The choice we make in this life will also determine where we'll spend eternity- Heaven or Hell.
