Neil Shubin's Your Inner Fish is one of the best science books I've read in a long time. Shubin manages to explain paleontology, human anatomy, evolutionary theory, and geology in terms the average semi-literate reader can understand -- and does it all seamlessly in a fairly small number of pages. Your Inner Fish had been on my "must read" list since it was published last year, and now I'm wishing I had gotten around to it sooner.
Shubin looks at various parts of the human body -- the hand, the bones of the inner ear, and others -- and explains how the fossil record shows those body parts evolving over time. It's fascinating. Who would have thought that the auditory ossicles started off as part of the jaw and moved over millennia?
While he's doing that, he's also throwing in personal anecdotes about the joys of field work (he's been doing his in the Arctic so the hazards he gets to deal with include polar bears), explaining geology, casually explaining genetics and mutations, and describing both how fossils form and how paleontologists decide where to go looking for them. It's a great book for anyone who has any interest at all in the how and why of science.
This book was one of those rare ones, too, in nonfiction where you pick it up and get sucked right in. It's actually fast, fairly easy reading. I zipped through it in a couple evenings. I spent actual money on this book instead of looking for it at the library, and it was money well spent.