So far, it's a pretty steep departure from the other books. My least favorite so far was The Silver Chair. It was just kind of dull. A Horse and his Boy was vaguely racist, or at least ethno-centric, but it was actually pretty good.
It's funny, though, how much C.S. Lewis is diluting Narnia as the books go on.
In the Lion, The Witch & The Wardrobe, Narnia was this magical other world that had giants and talking beasts and nymphs and centaurs, but no humans (sons of Adam and daughters of Eve).
In Prince Caspian, we find that Narnia has been invaded and taken over by humans, apparently descended by dimension-hopping pirates from across the sea. Oh, by the way, there's another country in this world where humans live.
In Voyage of the Dawn Treader, we learn about these other countries that have slaves and stuff, and are apparently also habited by people. And in these other countries, no one has ever heard of talking animals or giants or anything. Oh, and the REAL magical land is in Aslan's country, aka Heaven.
In The Silver Chair, we actually gain a bit of ground. In addition to Narnia, we also discover an entire underworld kingdom, and get a glimpse at an even deeper kingdom where there are rivers of fire that talking salamanders live in. That's pretty magical.
In A Horse and his Boy, though, we learn more about those other countries mentioned earlier. Not only are there other countries, but there's a vast empire of Arabic type people, and Narnia is only a tiny country on its northern border, barely worth mentioning.
And in The Magician's Nephew, we get the ULTIMATE diluting of Narnia's specialness... because now we learn that there are actually infinite (or at least hundreds) of other worlds, that you can access through The Woods Between the Worlds.
It's not really a big deal, but it is strange. Part of me kind of liked it more when Narnia stood alone, as a magical other world that you could escape into.