This is Book 5 of The Chronicles of Narnia.
In terms of Christian allegory and stories with morals, this book pretty much hits one upside the head with the Sledgehammer of God. There's no light hand in this story, to be sure.
Continuing the Narnia tale, Lucy and Edmund go back to Narnia, along with their cousin Eustace, who's the 1950's model for Dudley Dursley. He's a little shit, know-it-all, arrogant, lazy kid. We could call him a Well Actually and be accurate. He goes off by himself, turns himself into a dragon, learns his lesson through loneliness and loss of connection with all his human friends - why they didn't immediately kill the dragon flying over the camp? - and needs to be restored only through the grace of Aslan washing away his sins in special water.
And then he drops from the story.
There are a number of other smaller lessons learned, don't eavesdrop on others, turn towards God for strength, getting everything you want is a Bad Thing™, regaining your throne is as easy as walking into the top government house and declaring you are the king works every time even when you're a slave, and, for all the flat-earthers, Narnia is flat with the oceans having an edge.
One of my frustrations with the book is Lord Rhoop, who was found on Dark Island, where your dreams (literal dreams, including nightmares, not daydreams) come true. He escaped, yay he was rescued, and HE LEFT OR LOST ALL HIS MEN ON THAT ISLAND. They celebrate this guy's rescue, but he was a horrible leader, all of his men were dead or left for dead. This is a horrible result. None of the men who went out with the seven lords of Narnia were ever found. So, in Narnia, the only people who matter are named royalty?