This Time is Different by Carmen Reinhart and Ken Rogoff

Using their rolodex to the fullest, Reinhart and Rogoff assemble the most complete history of debt and GDP we have seen to date. Covering eight centuries, they show that when the outstanding issue of government debt / GDP exceeds 80%, governments start defaulting far more often than when below 80%.

The data and expository writing for the book exist online at http://www.economics.harvard.edu/files/faculty/51_This_Time_Is_Different.pdf.