That's my favourite Narnia book. "Out of the Silent Planet" is my favourite CS Lewis book.
(Incidentally, the Silver Chair shares a line with a short story by Damon Knight made famous by an episode of the Twighlight Zone. Both were published around the same time. Does anyone know who came up with the line first? I don't want to say the line, as it's a spoiler, but if you don't know what I'm talking about, you won't know the answer anyway.)
All time favourite: Good Omens or Hitchiker's Guide to the Galaxy