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.