The Eternal Frontier: An Ecological History of North America and Its Peoples | 
It's an amazing blend of geological, biological, and social history. It's concise and well-paced. It is an absolutely gripping story as Flannery relates the rise and fall of species and climates with such epic sweep that you feel like God watching it happen. Nothing like it. Stephen Gould would've been proud. If you only read one natural history book in your lifetime - this is the best summary you could choose
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