I'm following in Kathleen Krull's footsteps (see her post of 12/8, below) with a few more holiday book suggestions. Science is the subject of all of these titles, but it is addressed in a cultural context that makes these books relevant to a broad audience.
The first is The Demon-Haunted World: Science as a Candle in the Dark, by Carl Sagan and Ann Druyan. Sagan, who died in 1996, didn't always get the respect he deserved as a scientist and writer. Ironically, his successful efforts at popularizing science on television — most famously with the PBS series Cosmos: A Personal Voyage — probably contributed to a perception of him as a TV personality rather than a serious astronomer and author. Demon-Haunted World is an accessible, intelligent, and engagingly written look at how science works and why pseudo-science deserves its moniker. In it Sagan and Druyan logically and systematically debunk alien abduction stories, astrology, faith healing, crop circles, and many other irrational beliefs. More significantly, the book is a celebration of science and how it has freed us — some of us, anyway — from the suffocating weight of superstition. If you enjoy Sagan I also recommend The Varieties of Scientific Experience: A Personal View of the Search for God, also written with Druyan. The title is a reference to William James's The Varieties of Religious Experience: A Study in Human Nature.
Here's another: Unweaving the Rainbow: Science, Delusion, and the Appetite for Wonder (1998), by Richard Dawkins. The British evolutionary biologist was best known, until recently, for his book The Selfish Gene (1976). With the publication of The God Delusion in 2006, Dawkins has become something of a spokesperson for those with a skeptical, secular worldview. He's perhaps the most recognizable of the recent crop of authors critical of organized (and unorganized) religion. The God Delusion is an interesting read, but I think Dawkins's real contribution to the lay reader is his brilliance at explaining evolutionary biology in a lucid, conversational style and his ability to communicate something of the beauty and wonder of the natural world as it is perceived and deciphered by science. Unweaving the Rainbow is an unsentimental love letter to science. If you like it, try Climbing Mount Improbable (1996), which is as clear an explanation of the theory of evolution as you'll find.
Finally, a more recent title: Only a Theory: Evolution and the Battle for America's Soul, by Kenneth R. Miller (2008). There are plenty of books that convincingly refute the case for Intelligent Design, and the author does a nice job of that here (for a biologist, refuting ID doesn't require much heavy lifting). But that's not the main point. What makes this book interesting — and important — is Miller's argument that the battle over evolution will have scientific and social consequences far beyond the fate of whether or how one theory is taught. A serious read, but one that's hard to put down.