Gowers' Mathematics: A Very Short Introduction. This is in Oxford's series of "very short introductions" on a variety of topics (hieroglyphics, film, Rousseau,...), each of which I think is a tremendous challenge to the (invariably eminent) writer. Gowers dispenses with "anecdotes, cartoons, exclamation marks, jokey chapter titles," and instead plunges right into details without apology. The chapter on proofs is especially important for nonmathematicians to understand. The explanations of concepts (e.g., "dimension") are lucid, achieving clarity without compromising on technical accuracy. I read it in one sitting, which may dismay the author who must have labored over these small 160 pages, but which is a testimony of how smoothly he conveys his insights.