The Blind Watchmaker by Richard Dawkins
Although this is one of my favorite books of all time, I must admit that it has one very small flaw; namely, that it is far too much of a polemic to be what the author apparently wanted it to be, that is, a tool for persuading Creationists that evolution is in fact a fact. Indeed, Dawkins is so arrogant that even most fence-sitters on the issue are likely to find him a bit annoying.
However, this book is sure to be enjoyable reading for any biologist who is tired of taking lumps, or anyone who needs more debating tools. Dawkins covers many of the common variations on the Argument from Design: the Watchmaker analogy (as the title implies), the airplane-in-a-junkyard analogy, the problem of the eye, and so forth.
He also covers other topics, from his ongoing debate with Stephen J. Gould (also a writer worth reading), the use of computer models in evolutionary theory, and many other scientific delights.
For those more interested in Dawkins' own work, I recommend his other books, particularly The Selfish Gene and River Out of Eden. If you like a good controversy, though, definitely check out The Blind Watchmaker.
An unofficial Richard Dawkins page.