I have a bit of a theory. I think "The Voyage of the Dawntreader" is Lewis' grail legend. If that's so, then I'd hazard a guess and say that "The Silver Chair" is his "Pilgrim's Progress." -just think about the shape of Puddleglum's hat and the fact that he lives in the Fen Country and you'll see what got me thinking down this line.
That brings me to why I like "The Silver Chair" so much. When I was little, we had a children's version of "Pilgrim's Progress" that my mom used to read to me. I lived in New England and the Christianity I was raised with had a heavy tinge of Calvinism and the Puritans. Puddleglum spoke my language. We New Englanders are cold and formal people, with more than a little well-relished pessimism. As far as Christendom goes, there's a reason why we're exiled to the ecumenical fens (those of us who haven't gone Unitarian). Still, at the uttermost, there's a vein of steel there that will stand true when all else has failed. One of my favorite images from Narnia has to be Puddleglum, when all argument has been defeated and his friends have given up, defying the green lady and her empty world right before he thrusts his bare foot into the fire to stamp it out.
People don't really like the Puritans, but Lewis reminds us why they are there. They keep going when the fair-weather Christian-hedonists have given up. A marsh wiggle may not be the most pleasant companion, but when something sets itself up against the knowledge of God, no matter how high, then they will be there to defy the tyrant to the last. Chesterton said that the sanity of Christendom was to assign a proper place for everyone. Lewis' Narnia is an outworking of that idea. So let's hear a cheer (but a very proper one) for the Puritans.