The Royal Society of London announced 13 finalists for the 2009 Royal Society Prize for Science Books to celebrate the best of 2008's new popular science writing for a general (adult) readership.
I've not read any on this list so I can't speak for their quality, but in general I really enjoy general audience science books, especially if the author can tell a good story.
Here's a list of a few of my favorites in no particular order:
The Omnivore's Dilemma by Michael Pollan
A Sand County Almanac by Aldo Leopold (more a journal of his observations about the environment than an actual science book, but still one of my favorites).
Monster of God by David Quammen
A Short History of Nearly Everything by Bill Bryson
In Defense of Food: An Eater's Manifesto by Michael PollanA
Why We Run: A Natural History by Bernd Heinrich
The End of Nature by Bill McKibben
and pretty much any book by Richard Feynman!
What's yours?