Many books can be described as "good"; but it is the rare book that really deserves the title of great. In my opinion, The Beak of the Finch deserves to be considered in the latter category. It is not simply a good book worth reading; it is a great book that should form a model for scientific nonfiction for decades to come. It has won half a dozen awards in both science and literature and deserves to win many more.
Writing in a very accessible style that is nonetheless not dumbed-down or oversimplified, Weiner describes the time he spent with Rosemary and Peter Grant, two eminent experimental evolutionary biologists who also happen to be a married couple. He listens to the scientists and faithfully reproduces their research, rather than narrowing it or sensationalizing it as many journalists would.
Yes, experimental evolutionary biologists. The Grants don't simply analyze evolution based on paleontological finds or mathematical theories like most evolutionary biologists—they go out in the field and watch evolution happen, carefully observing and controlling for different variables in actual field experiments. The Beak of the Finch is about this marvelous empirical project.
Before the Grant project, many biologists thought that evolution could not be observed on this timescale, particularly not among what we call metazoa, that is, complex multicellular animals, animals in the usual sense of the word (rather than the much broader concept of Animalia biologists use). The thinking was that evolution could only occur on timescales of at least thousands if not millions of years—but through years of painstaking research, the Grants observed it over years and decades.
It's difficult to overstate the importance of this result; it's a difference of between two and six orders of magnitude. It would be as if we suddenly discovered that snails were outpacing Olympic sprinters.
There is a somewhat informal unit of the rate of evolution, the darwin, which is a 1% change in some trait over 10,000 years; named by Haldane in honor of Darwin, this is about the rate at which Darwin believed evolution should occur. The Grants observed changes on the order of 1% every year—meaning that those finches are evolving at a rate we'd have to measure in kilodarwins.
This fundamentally changes our understanding of how evolution happens—it's not a slow, gradual process in one direction, but a constant rapid oscillation around a slowly-changing equilibrium. Animals can therefore potentially adapt to environmental changes much faster than previously believed—which makes mass extinctions that much more terrifying.
I cannot recommend this book emphatically enough. If you read one book on evolution, read On the Origin of Species. If you read two, read The Beak of the Finch. (If you read three, read The Selfish Gene.)