I have met Bill Cronon a few times, and have read several of his books. Josh Marshall mentions his first book, Changes in the Land--and it is indeed impressive. But for me, his book on Chicago (Natures Metropolis) was not only the book that informed my dissertation, but struck me as one of the smartest books I had ever read. A book, I would add, that I read thinking, "there is no f-ing way that I could ever write this book."
Cronon wrote a great NYT op-ed this last week that placed the Wisconsin attack on unions in a historical context. On his blog, he had also started asking questions about who was pushing this attack on unions. He evidently struck a nerve, because the Wisconsin GOP has requested Cronon's university email under their open records act.
Here is Cronon's lengthy and thoughtful response. Worth the read. Abusing Open Records to Attack Academic Freedom | Scholar as Citizen