Have just finished reading The Watchers by Stephen Alford (Professor of Early Modern British History at the University of Leeds).
It is an academic study of espionage in the reign of Elizabeth I.
It is a dense book, and written for an academic audience, but worth persisting with (I read it at a rate of ten pages a day, which was just enough to stop my head from spinning).
We are used to thinking of the first Elizabethan Age as a golden period of expansion, innovation and dazzling culture (literary, in the visual arts, architecture etc).
This book reveals how threatened the Elizabethan state was (by enemies within and without) and the necessity for constant vigilance to outwit the assassins, the seditions, the acts of terror.
"The weapons they used were espionage, relentless interrogation, surveillance, the suppression of dissent, robust treason law, torture and propaganda."
Which rather sounds like the situation we are in today (except for the torture bit - the British security forces are not involved in torture).