The Chronicles of Narnia: The Last Battle by C.S. Lewis

The Last Battle takes place once again in Narnia, following the point of view of King Tirian, the last king of Narnia after hundreds of years. Meanwhile, an old ape named Shift decides to manipulate his companion, Puzzle the donkey, and disguise him as Aslan to get everyone to do his bidding. Tirian and his friend, Jewel the unicorn, have been captured by the Calormenes, enemies of Narnia. He is rescued by Eustace and Jill, and the group, along with Puzzle, who is now revealed as the fake Aslan, fight the Calormenes. The group walks through a stable door and find themselves in Aslan's country, along with everyone who has been in the past stories: Peter, Edmund, and Lucy Pevensie, Puddleglum, Caspian, Rilian, and many others in previous books. There, they all find out that the reason Eustace, Jill, and the Pevensies made it to Narnia was because of a railway accident, and they are now in what the book perceives as Heaven.