Quick, name a Narnia book that isn’t The Lion, the Witch and the Wardrobe. Hint: You’ve got six options.
Long story short: there are seven Narnia books, about every other one is good, and you’ve probably only ever read The Lion, the Witch and the Wardrobe. And what’s why I’d like to bring to your attention my favourite Narnia book, The Horse and His Boy. Despite being one of the lesser-known Narnia books, it is actually my favourite in the series, by virtue of being of the weirder ones.
Horse and His Boy doesn’t really fit in with the rest of the series, being set outside of Narnia proper and not directly related to the wider story arc of the rest of the series, and therefore has the most capability of standing on its merits as an individual story. It’s also a much smaller, more intimate story in terms of number of characters and scope than the rest of the Narnia books.