The Horse and His Boy
The Horse and His Boy is a book by C.S. Lewis and is the fifth in the Chronicles of Narnia. In the chronology of the world of Narnia it comes during the later part of the events of The Lion, the Witch and the Wardrobe. It was originally published in 1954 and dedicated to Lewis' stepsons, Donald and Douglas Gresham. 
This book tells the story of Shasta, a young boy who has lived with a fisherman called Arsheesh. He learns one night that he is not in reality Arsheesh's son at all but had actually floated down from the river in a boat with a dead man in it. He overhears this and learns also he will be sold as a slave to a Tarkaan (a great Calormene Lord.) He then meets a talking horse called Bree, who used to live in Narnia long ago. They run away for "Narnia and the North."
Shortly afterwards they meet two other runaways. One of them is called Aravis, a young girl running away from a forced marriage. The other is a mare, who, like Bree, is from Narnia. They join together and cross the lands of Calormene until they finally reach the capital city of Tashbaan. Here they unfortunately are separated when Shasta is mistaken by Narnians for Prince Corin of Archenland (another northern country.) Here Shasta (still thought to be Corin) meets Queen Susan, King Edmund, and Mr. Tumnus from The Lion, the Witch and the Wardrobe. He hears their plans to leave Tashbaan and meets the real Prince Corin of Archenland whom helps him escape.
He then heads for the desert where he reunites with Aravis and the horses. he learns from Aravis that Prince Rabadash who infuriated by Susan's refusal of his marriage proposal decides to secretly attack Archenland and then Narnia. They race across the desert to Narnia but, due to other reasons, Shasta ends up being the one to bring the message to Archenland of Rabadash's invasion. He then finds himself alone and lost and starts moaning his losses to himself.
This does not last long until Aslan, the great lion, appears and shows him how fortunate he really is. He then comes to Narnia where he brings news of the invasion of Archenland. He then takes part in the battle of Anvard where most of the calormenes are slain and Prince Rabadash imprisoned. Then we return to Aravis and the horses who have been waiting at a Hermit's lodge for the time being. They also are confronted by Aslan and told of Shasta's coming. Shasta comes and explains that he was truly a Prince of Archenland and twin brother of Corin, Cor. In fact, one day he will be its king.
He takes them to Archenland where one of the last things done is the judgment of Rabadash. They give him terms but he only mocks them. Then Aslan, the great lion, appears and turns Rabadash into a donkey until he appears in Tashbaan during the autumn feast! Cor and Aravis long afterwards marry and they then live happily ever after.