I am very interested in learning the goings on of intellegence agencies. Historical or current is fine with me
Look for James Bamford's books:
The Puzzle Palace: Inside the National Security Agency, America's Most Secret Intelligence
Body of Secrets: Anatomy of the Ultra-Secret National Security Agency
The Shadow Factory: The NSA from 9/11 to the Eavesdropping on America
I've read and enjoyed the first two, didn't read the third, yet. Remember the first rule about fight club.