The Horse and His Boy is the third book in the Narnia sequence, chronologically, and that’s the way I’ve always read the books. It was one of my favourites as a child: Calormene was so different to Narnia, and Aravis is so badass, and there’s so much adventure!
As an adult, it kind of felt a bit hollow, though… it feels like an afterthought: most of it doesn’t even happen in Narnia, and the events aren’t super-significant to Narnia as a country. It does give a bit more depth and breadth to the world, insofar as you find that rings true, but it’s also a mess in terms of yanking stuff from real-world cultures and demonising the fictional result (to some degree or another — I acknowledge that the existence of Aravis makes it more complex, and e.g. Lasaraleen isn’t actually terrible as a person).
It’s still a fun adventure, and there are some lovely descriptions… and I wish there was more of the Hedgehog and the Rabbit and the other talking animals Shasta meets when he arrives in Narnia, to be honest. The dwarves he was staying with get rather dropped, despite their kindness to and care for him!
Alas, though, I think this one I’ve just sort of grown out of, on the whole.