Pullman at his best - simple, but with layer upon layer of depth. I have to say, with some embarassment, that I did not spot the "nature of the beast" until almost the very last page. I cannot bring myself to say what it was, either, because I enjoyed the surprise so much that I want others to read it and have that experience. All this is far from saying that the rest of the book was anything but a joy to read, with unexpected action leaping from each page.
As always, Pullman turns the world as you know it on its head - to a world that's, in many ways, more real than our own.