Banger book. I've always found Mackie's Queerness argument to be sort of weak because of how poorly and vaguely he states it. Olson clears all of that up, targeting the argument precisely and discussing its implications in a clear and well-defined way. While I disagree with some of his later elaborations (particularly when he tries to save hypothetical imperatives from queerness), this is still a vast improvement on Mackie's argument.

EDIT ~ I year later, I am bumping this down to a 4.5. Still a good book, but I don't think it belongs to the very best.