Sad news. His book The Keynesian Revolution (1967) catapulted me from the point where my relation to The General Theory was that it was a book I read and was broadly impressed with in 2006 because I wanted to give it a more thorough read after the spot-treatment I got in my history of thought class to the point where Keynesianism was a major intellectual interest. It's really a phenomenal book that gives you the analytics, an appreciation of the intellectual response, and a good sense of the intellectual history of Keynes. It was also written before the rational expectations revolution or even Friedman-Phelps really came into the picture, so it is a nice statement of Old Keynesianism. That's not to say New Keynesianism is some kind of problem (as some would have it). I don't think that. It's just a nice unalloyed picture of Keynes as we saw him from the vantage point of the 1960s.
Klein is the epitome of big macroeconomic models that would later come in for such abuse for having neither the microfoundations demanded by the Lucas critique nor the elegance you expect in modern reduced form macroeconometrics. He had a hand in a lot of the major efforts of this sort, at the Cowles Foundation, at the University of Pennsylvania, and at Brookings. This is what he was eventually cited for in his Nobel prize.