For this one, I thank Jon Stewart's The Daily Show for this one. He had Elizabeth Kolbert on the show and I was really interested in the book. I recommend following the link to go to the interview!
The Sixth Extinction covers the mass extinction happening right now to many plants and animals. There have been 5 extinction events previously, with the dinosaurs being the newest (relatively speaking). The cause of the sixth event turns out to be.... us.
Kolbert lays out the evidence that humans are, purposefully or not, pushing species right off the ledge of extinction. She lays out different issues in each chapter, ranging from the ever present but still poo-poo'd climate change, ocean acidity, coral reefs breaking down to how modern humans wiped out Neanderthals (after having sex with them).
It's not a horribly in-depth book, so you don't need a science degree to understand it, but it delves enough into each scenario to make you worry about the future of the earth. As one scientist noted we are liable to wipe ourselves out along with other species. And that is pretty scary.