Oh wow, he wrote Chesterton's liar-lunatic-Lord gambit right into that book! In case you didn't know, Lewis (and Chesterton before him) had used that argument for the credibility of Jesus: either he was lying, he was crazy or he was the Messiah like he said he was. It never seemed like a good argument to me, since option 2 is so unclear; it seems based on the idea that you are either a raving and obvious loony, or you are perfectly sane. As we know, however, the line between those two states can be all too blurry sometimes...
(BTW, I am told that Chesterton actually used this not to argue for Jesus' general divinity but against the "he was a wise teacher but not God" line of argument. That makes more sense than the way Lewis used it.)
I didn't know that was Chesterton's gambit; Lewis had written about it in "Mere Christianity." That said, it never really clicked in my brain until I read your comment. Lewis put a LOT of symbolism into his Narnia books, yet the skill with which he did so continues to impress me -- two decades later!