The Sixth Extinction: An Unnatural History By Elizabeth Kolbert

This is an excellent description of the current state of knowledge about the five mass extinctions in the history of the world.  The premise is that we are currently witnessing the "sixth extinction" as species are becoming extinct faster than any other time in history, except the other five extinctions.  In some ways the current extinction is causing species to die faster than some of the others.  Elizabeth Kolbert presents hundreds of very sad examples of the loss of species.  Her examples show how so interconnected each species is with so many other species of plant and animals.  She also did a lot of personal travel and research to provide good, first-hand descriptions in her examples.
There is also a good youtube video that summarizes very well the details presented in the book.  See this link: http://youtu.be/z9gHuAwxwAs