I'm a big fan of Quammen's writing, having read his excellent The Song of the Dodo and Monster of God. This book is equally well-written, but is really a primer, rather than a full-blown book. I wish Quammen had written a full-blown book, because what he did write, as far as it goes, is very good indeed.
This book describes and explains Darwin's procrastination over publishing On the Origin of Species. Quammen starts the story after Darwin returns from his Beagle Voyage. It explains how he thought up his big idea of Natural Selection shortly after the voyage, then delayed publication for 20 years, becoming a world expert in barnacles in the meantime. Only when he was in danger of being scooped by Alfred Russel Wallace did he finally rush to publish.
Any Darwin groupie will already be fully familiar with this story, but Quammen's version is a nice, succinct read.
Finally, a bit of a rant: this book appears to have two entirely different titles in the UK and the USA (why do they do that?). The version I read is called, The Reluctant Mr Darwin (which appears to be its American title), but there is also a version doing the rounds in the UK entitled The Kiwi's Egg. Totally bloody infuriating!