If you haven't read it recently or (perturbed intake of breath) at all, you owe yourself. It's a brilliantly cast parable, or "modern fairy tale" as Lewis puts it, of the timeless war of Arthur against barbarism -- this time, the barbarism of Modernism. You don't at all have to have read Out of the Silent Planet or Perelandra, the first two parables in the "Space Trilogy" series, for the book to work. Think of it as "C.S. Lewis' Scouring of the Shire," with a whole lot more quiet psychological horror up front. If I were forced to put it in a single genre, it would nonetheless be that of the supernatural horror novel -- my poor defunct copy was, as it happens, shelved with the horror paperbacks.
And re-reading it, I once more squirmed as Studdock slowly sells everything for nothing -- the resonance with good old Edmund from the Narnia books was louder for me than usual, no doubt thanks to the movie -- and I enjoyed seeing the book through ever more Tim-Powersy eyes. (The eldil and Macrobes are *perfect* Powersian entities, and I don't doubt that Powers read Lewis back in the day. The spinning room is a dead giveaway.) And I delighted again to see the lightning-flash of Lewis' eye for moral character and his gift for the cutting phrase. (That said, one or two of the more, shall we say, robust pronouncements from Ransom, the Pendragon, may well grate with our delicate breed of modern readership. Suck it up. None of the argument can be dismissed as sloppy, and not much more of it can be dismissed as bad writing.) I've become less Platonist than I apparently was the last time I read it, because I think I jibbed a little more strongly at Lewis' Platonism this time, but there's no denying that it makes a fascinating supernatural element.
In short -- terrific book, like I need to tell you that. If I need to tell you that, well, read it. But try to get a copy less than 30 years old, because you'll definitely want to re-read it again and again.