[Symmetry]

Really I'd recommend reading chapter 1 of The Wages of Destruction. The book is really on the economics of WWII but it summarizes the modern consensus on the Great Depression as a starting point and since it fits everything into one chapter ends up being very readable. Mostly it was contagious deflation mediated by the gold standard with gold hording causing deflation causing more hoarding. Countries on the silver standard like China were essentially unaffected. Countries that went off the gold standard early like the UK or Japan recovered early. The Wikipedia article is mostly a history of how economists' understanding of the Great Depression has changed over the decades rather than a description of where it stands now but it works pretty well.

https://en.wikipedia.org/wiki/Great_Depression#Mainstream_ex...

EDIT: A while ago Krugman had a very good piece trying to explain how deflation can cause problems. Different sorts of economists often disagree about whether its interest rates or aggregate quantities or expectation traps or whatever that's the most important aspect of tight or loose money but they all agree that these things are real and meaningful and generally explain depressions. http://www.slate.com/articles/business/the_dismal_science/19...