The Watchers, a Secret History of the Reign of Elizabeth I by Stephen Alford (nonfiction)

Interesting, illuminating, and the first three chapters were gripping.  They were characterizations of three real spies working for Elizabeth's government.  By the end of the book, I understood more about the real threat to Elizabeth, and more about how her own government contributed to the culture of paranoia concerning that threat.  There was a full explanation of how they caught Mary Queen of Scots breaking specific treason laws.