Without Cloak or Dagger: The truth about the new espionage -

Although published in 1974, it's still the best book on humtel (i.e., human intelligence). Covers everything: recruitment, training, case officers, desk officers, security officers, cutouts, analysts, the difference between intelligence officers and spies, dummy corporations, front organizations, the use of journalists and academics, difference between intelligence and espionage (and counter-intelligence and counter-espionage), the difference between intelligence gathering and law enforcement (and why their goals often conflict), organizational structures of intelligence agencies, career path of a typical CIA employee, brief history of the OSS and CIA. If you want to know what spying IS, and HOW it works, this book is a great start.