"Guns, Germs, and Steel" counts as one of those eye-opening books that correlate things you hadn’t thought about before, just like "Godel, Escher, Bach" a while back. I enjoyed it immensely, and I was looking forward to whatever Jarred Diamond was cooking up next. Which turned out to be the present book, Collapse.
Guns was about how European societies dominated the world because Europe benefited from climatic benefits that no other area of the world had. While the premise was deeply flawed (China had none of the benefits, rose much faster than Europe, and then declined), it was a very interesting read, since it pretty much said that any culture would have made it to the power of Europe if it had had the advantages that Europe enjoyed.
Collapse focuses on societies in the past, present, and future that have collapsed, are collapsing, or will collapse. The book is entirely different than its predecessor, because it expounds a cause that is not rooted in the past, but that affects the present, which of course puts the whole book in the category "controversial."