Jadis's description of her sister as someone even worse than herself. Apparently the sister broke the pact that neither side should use magic in the civil war, "poured out the blood of her armies like water" and would have been a worse ruler than Jadis herself. Was Jadis once more moral than her sister and had to become the tyrant she is to match the sister's forces? Or is Jadis exaggerating her sister's evil in order to justify herself to the children? Jadis says that her sister refused to surrender, implying she threatened her with the Deplorable Word; did the sister know Jadis was going to use it or did she think Jadis wouldn't go through with her threat?
Uncle Andrew claims his godmother was descended from fairies and got the magic rings from Atlantis. Was Atlantis a real place in Earth's ancient history? Or was it another world whose inhabitants came to Earth?
Author's Saving Throw: Polly is more heavily involved in the plot and has just as many adventures as Digory, which is a response to the Stay in the Kitchen attitude Lewis demonstrated in earlier books. This is believed to be influenced by his marriage to ardent feminist Joy Gresham. Notably Aravis of The Horse and His Boy is a strong female character, and Lucy Pevensie fights in the battle towards the end - as does Jill Pole in The Last Battle. The only reason Polly isn't involved in the mayhem when Jadis acts up in London is because she's being punished by her parents.
Evil Is Sexy: Both Digory and Uncle Andrew are impressed with Jadis' beauty; Polly on the other hand is quite immune.
Fanfic Fuel: The Wood between the Worlds. There are countless, possibly infinite portals to other worlds, but we only see three: Earth, Charn, and Narnia. This book also reveals that, in Earth's ancient history, Atlantis was real and Fairies existed, neither of which is ever mentioned in any of the other books.
Fandom Berserk Button: Don't call this the first Narnia book. While the earliest in the series' chronology, it was written second-to-last and the multitudes of Call Forwards are lost on anyone who starts with it. And it really doesn't help that some editions label it as number one.
The description of the garden where Digory gets the apple is a direct nod to how John Milton describes the Garden of Eden in Paradise Lost.
The garden also has parallels to the Garden of the Hesperides in Greek mythology. Digory is sent to retrieve an apple from the garden like Hercules, and Jadis steals one from the tree just like Eris stole the Apple of Discord. What's more is that the Apple of Discord started the Trojan War, and Narnia's first queen is called Helen.
Uncle Andrew's godmother is an allusion to Morgan Lefay from Arthurian mythology.
Harsher in Hindsight: Queen Helen is said to look beautiful in her simple attire, and would have looked tacky if she'd put on her best outfit. Years later Queen Susan becomes no longer a friend of Narnia because she becomes interested in "nylons and lipstick and invitations".