The Horse and his Boy (1954) is a Narnia story by C S Lewis which while definitely part of the Chronicles of Narnia is different to the other books. It is all set in the land of Narnia (well Calormen and Archenland.) The events happen coevally in Narnian time with the last chapter of "The Lion, The Witch, and the Wardrobe".
The plot has Shasta, a slave in Calormen, fleeing to Narnia and freedom with his horse Bree. Or as the title suggests it might be seen more as Bree flees to Narnia and North taking his boy Shasta with him. On the way they have adventures, meet fellow travellers, and defeat a nefarious plot. Engaging writing, and C S Lewis depicts a different world in the Asiatic style Calormen.
There are several key themes in the book. Aslan is depicted as working his purposes out in people's lives, even though it may not be obvious at the time that he is so doing. Some parts of our lives can only be understood in hindsight.
Another theme is retribution. Aslan wounds both Shasta and Aravis for actions they have taken earlier. I'm not so sure how Christian this theme is as Aslan has not applied the same logic at other times. And part of the Christian message is that we haven't redeemed ourselves, it is by grace we enter. So this doesn't sit perfectly with me.
Yet another theme is the lies people tell themselves and others. So Bree has in the end to come to terms with just being a horse, nothing more nothing less. The fisherman who looks after Shasta in the beginning lies about what's beyond the hill as a way of not having to know what is beyond the hill. Prince Rabadash says he wishes the Tisroc will live for ever which he both doesn't believe, and certainly doesn't hope for.
An engaging read.