The Demon Haunted World, by Carl Sagan. I consider this to be one of the most important books a person could read. It is a brilliant introduction to the scientific method, critical thinking, and the problems that can arise when skepticism is not applied. Well written, unputdownable and very funny in places (e.g. one of the responses Sagan received to a magazine article discussing the poor standards of science education in America read "Not a Americans are stupid We just rank lower in school big deal."(sic)) If I ruled the world, everybody would be forced to read this. =)
The Selfish Gene, by Richard Dawkins. A deep study of the "selfish gene" theory of genetics and biology. Essential reading for understanding this aspect of the theory of evolution. It's one of those rare books that makes you go "Wow..." when you finish it, and all living things are seen in a slightly different light.
The Blind Watchmaker, by Richard Dawkins.An excellent introduction to evolutionary biology and natural selection. Plenty of discussion on the way things that appear to be thoughtfully designed can arise from the "blind", natural process of evolution. If you're at all interested in evolution, you need to read this book.
The Meme Machine, by Susan Blackmore. In the Blind Watchmaker, Dawkins introduced the concept of the "meme", as an example of a self-reproducing object that undergoes natural selection, to demonstrate that evolution applies to non-biological entities as well as just genes. Blackmore expands on the idea and brings it up to date. This book will challenge (and possibly change) the way you think about thinking...
(It also has some interesting implications for religions)
Why People Believe Weird Things, by Michael Shermer. A fairly in-depth look at a handful of strange beliefs, including UFOs, Holocaust denial and evolution denial (creationism). Interesting and amusing.
The Birth Of Time, by John Gribbin. A history of how humans found out where they are in the universe, studying the changing views of the size and age of the solar system, to the galaxies and stars, trying to track down the elusive Hubble Constant all the while. If you're interested in astronomy/cosmology and the development of science, you'll like this.
Unweaving the Rainbow, by Richard Dawkins. Dawkins makes a brave attempt at demonstrating that science can greatly enhance the beauty we see in the world around us. The title comes from the complaints made when Newton first broke sunlight into the spectrum, "unweaving" it and upsetting the poets and romantics who objected to nature being reduced to mere scientific phenomena. Dawkins argues that in gaining greater understanding of the world we can appreciate it more and feel even more wonder at it than ever before. If you have any artist/poet friends who object to scientific enquiry, read this.
Life - an unauthorized biography, by Richard Fortey. A natural history of the world, from the development of the first single-celled life forms, through the different stages through the pre-cambrian, cambrian, right up to modern organisms. Quite heavy going due to the vast amount of subject matter (i.e. all of life) but fascinating nonetheless.
Evolve Or Die!, in the Horrible Science series. An introduction to the fact and theory of evolution and natural selection for youngsters. My son read it at age eleven and thought it was "cool". A stocking filler for your creationist friends' kids. =)
Climbing Mount Improbable, by Richard Dawkins. A fairly gentle introduction to evolutionary biology, explaining the concept of evolutionary "landscapes". Relatively easy reading, compared with "Watchmaker", but well worth it.
Maybe Yes, Maybe No, by Dan Barker of the Freedom From Religion Foundation. Similar format to "Maybe Right, Maybe Wrong" (below). A guide for young thinkers, introducing children to critical thinking and the scientific method.
Maybe Right, Maybe Wrong, by Dan Barker of the Freedom From Religion Foundation. A guide to thinking about and understanding morality, for younger readers. Explains how we can decide whether something is right or wrong for ourselves using reason, experience and compassion. Much more helpful than "Thou shalt not..."
A Brief History Of Time, by Stephen Hawking. Quantum physics, evaporating black holes, right-angled imaginary time and lots of cosmology. Heavy going, but you get through it all eventually and realise that the hype was justified.