Speer was hardly a disinterested commentator on this, and I seem to remember his book has more than a few problems, and isn't Keegan's book rather general in its scope?

My current source is the relatively new economic history of Nazi Germany, The Wages of Destruction by Adam Tooze (http://www.amazon.com/The-Wages-Destruction-Breaking-Economy...). It's very good, and shows how economics were vitally important from beginning to end.

E.g. part of the motivation in invading the Soviet Union was to address the crippling loss of productivity in their "Greater Western European Co-Prosperity Sphere" from the end of British coal imports. They needed to get enough food to the miners of the Lowlands etc., and the scheme was to starve the USSR cities and redirect the agricultural surplus east. Of course it didn't work out, but they did have a plan that wasn't entirely irrational.