I've heard good things about Freakonomics, but have not read it.
I have most recently read Out of Thin Air by Peter Ward, which looks at the theory that changes in oxygen levels led to the evolution of various body plans. More specifically, in times of low oxygen, a greater diversity of body plans appeared, while during times of high oxygen, a greater diversity of organisms appeared. An interesting read.
Currently, while I am reading Spindle's End, I am also reading The Scientific American Book of Dinosaurs ed Gregory Paul, which is a series of essays about current theories surrounding dinosaurs.
One of my favorite non-fiction books is A Fish Caught in Time by Samantha Weinberg, about the coelacanth...it follows the discovery of the first specimen through current investigations of the fish.