It is a terrible thing that Lewis’s publishers posthumously decided to issue the Narnia books in chronological order of the events, rather than in the original publication order. They claim it was Lewis’s wish, but the evidence for that is extremely flimsy — a letter he wrote to a child fan, apparently answering a question about the order of the events in the books.
The fact is, nothing else but The Lion, The Witch, and the Wardrobe could be the beginning of Narnia. It is the well-spring, the source; and only the most tone-deaf of editors could think otherwise.
Two things set LWW apart from the other books. First, it is the only one in which Narnia is not populated with humans. It is a world that belongs to the animals and tree spirits and mythological creatures. I think, at some level, many readers continue to believe they are in this Narnia throughout the books, despite all evidence to the contrary. But starting immediately with Prince Caspian, we are in a world where humans not only live but dominate. There is Telmar, Archenland, Calormen, Terebinthia, the Seven Isles, and the Lone Isles, all populated by humans.
A hand is waved, in Prince Caspian, at the idea that all humans in this world must have come from Earth (hence the Telmarines are descended from people who stumbled through a portal), but essentially, all the countries of the Narnian world except Narnia itself have an indigenous human population. (The earth couple that become the first queen and king in Magician’s Nephew? Apparently their offspring populated every country except Narnia!) And even Narnia proper falls into human hands, with the Telmarines who choose to remain forming a large percentage of the Narnian population thereafter. It’s hard to say whether this makes Narnia lose part of its charm, or whether it takes on a different kind of charm. (Certainly for fans of medieval European culture, it’s a win.)
The second way in which LWW stands out is in its lack of internal consistency, which might seem to be a flaw but instead is arguably the heart of the book. This is a land where Nordic dwarfs rub shoulders with Greek fauns, where Father Christmas makes an appearance defying all logic, and where the beavers eat ham and onions and potatoes and bread and butter and marmalade and tea, with (as Laura Miller points out in The Magician’s Book) no pigs, no cow, no plowed fields, and no climate for growing oranges or tea. None of that matters, in the first book. It is all sheer imagery, evoking a magical world that suits one’s heart’s desire. The beavers represent snug, industrious, thriving rural folk; of course Mrs. Beaver has a sewing machine and Mr. Beaver has tobacco.
It is later, when Lewis tries to develop a consistent history, geography, economy, and cosmology for Narnia, that he gets into trouble, and the inconsistencies become a problem instead of part of the charm.