This book focuses on the ever-evolving zone of parallel and overlapping control between the executive and legislative branches over the emergency or war powers of the federal government. It's a slippery topic, but Keynes's history is solid, and his analysis is clear and rigorous. The book draws its title from a phrase used by Justice Robert Jackson in his concurring opinion in the Youngstown Co. v. Sawyer case of 1952: between the powers exercised by the President on the basis of exclusive executive authority, and those exercised by Congress on the basis of exclusive legislative authority, there is a "zone of twilight" in which the two branches have concurrent authority, or in which the specific authority is unclear.

Highly recommended for anyone curious about the authority by which our current commander-in-chief has declared a state of emergency, suspended the rights of habeas corpus for thousands of non-citizens, and invented strange military tribunals to try potential suspects in what looks like it might be a never-ending war against terrorism.