Every Living Thing: Man's Obsessive Quest to Catalog Life, from Nanobacteria to New Monkeys by Rob Dunn (Smithsonian, 2008)
This is a book that does what great science writing is supposed to do - explain the universe while enthralling the reader. In exploring how the definitions and scope of the living world have been expanded over and over again by dedicated researchers, Rob Dunn gives us compelling portraits of biological scientists who have proposed "crazy" theories, made inconvenient observations, and otherwise risked their reputations and sometimes their lives in the pursuit of knowledge. Dunn shows that, while being laughed at by a majority of one's scientific colleagues is no guarantee of being right, it's far from a surefire indicator of being wrong.
One of the themes I push hard in my blog and my books on zoological discoveries and mysteries is that we don't know all the animals in the world, not by a long shot, and it's not just the little arthropods that are still being discovered. Dunn explores this theme and many related ones here. He anchors the book in his own experiences in the Amazon, where no one has any idea of the number of animal and plant species present, let alone what to name them. (He also passes along the intriguing story of what may be an unknown and very large type of spider monkey, which is intriguing because cryptozoologists have collected numerous reports of something with the same description.)
Dunn closes with the fascinating discoveries being made in the "deep biosphere" - the microbes that live miles beneath the Earth. Life on the surface, he notes, may in some ways be the exception.
This is a profound work, not just concerning the biological sciences, but concerning science as an enterprise.