The Horse and His Boy is probably the least essential of the Narnia
books (in its chronological narrative), but it's a rousing adventure
nonetheless, distinct by virtue of it being from the viewpoint of
someone who lives in the world of Narnia, but was not raised Narnian.
The
protagonist, Shasta, has a ruthless Calormene father and steals away to
Narnia on a talking horse, Bree. Bree turns out to be one of Lewis'
strongest characters, a brave, haughty friend who is as flawed as any
human. Along the way they combine forces with a young lady, Aravis, also
escaping on a talking horse. They eventually cross paths with Aslan and
the grown Pevensie children. Even old Tumnus makes a guest appearance.
For
those who find satirical jabs at other cultures intolerable, The Horse
and His Boy may be what's now euphemistically called problematic.
Otherwise, it's a quite enjoyable and suspenseful romance.

I'm in the minority when I write that I enjoyed the second Narnia book C. S. Lewis wrote,
Prince Caspian (1951), better than the first, The Lion, the Witch and the Wardrobe (though the initial inspired sequence of Tumnus under the lamp post, holding a package under his arm, may be the most resonant image of the entire series). Prince Caspian has classic political intrigue, as Caspian, suppressed by the usurper to the throne, Miraz, doesn't realize it's he who is destined for the throne.
Prince Caspian takes place 1,300 years after The Lion..., but only one year in British time. When Caspian calls back the four Pevensies, they're still children, but up to the task to battle for the soul of Narnia. As with The Lion..., Lewis creates wonderful characters, morally shaded. I especially appreciated the scenes of the gods Bacchus and Silenus which, with their intimations of the irrational and primal, create a thematic British link between the crazy modernist ending of G.K. Chesterton's The Man Who Was Thursday (1907) and Patrick McGoohan's exuberantly surrealistic ending of The Prisoner (1968). In any event, Prince Caspian is worth the reading, and rereading. The illustrations by Pauline Baynes are also delightful.