Shackley's a hard man to write a biography for, in that there's only so much *there* there, but Corn does a good job getting to the heart of the man and, more importantly, of the CIA in its figure-it-out-as-we-go first couple of decades.
Corn is no conspiracy theorist; he lands where many other tales of the CIA end up -- that American Intelligence was often as inept as it was criminal or harmful; how the Cold War mentality made allies or enemies of the world's nations with no in-between, and how that mentality combined with a "just following orders" moral compass excused nearly any otherwise inconceivable actions.
Highly recommended.