I got this book because I found the portrayals of Walsingham and Burghley in the films Elizabeth and Elizabeth I so fascinating that I wanted to read an account by a historian about what they represented and did. In the film, they combined politics, courtly intrigue, and the brutal tactics of espionage and war against the backdrop of the Reformation in the late 16C, with fascinating and cunning enemies.
The book starts off magnificently, with a recap of Elizabeth's reign and a sketch of the general situation of the Reformation in Europe. I enjoyed it very much for the glimpse it offers into statecraft as absolutism was coming into full flower. The book is fun in that it illuminates with great clarity what personalities in the film versions were really like, particularly in their more shady dealings. While this is available in the many excellent bios of Queen Elizabeth, the focus on these aspects is exactly what I was looking for. Essentially, Walsingham and Burghley created a network of paid informants that provided crucially important information about assassination plots and military threats to Britain from the catholic powers.
However, when it gets to the details of how the spying was done, it bogs down in detail that is of little interest to lay readers - I found how the spies were vetted and precisely how they accomplished what they did to be of lesser interest - and they form the bulk of the book. While the book does not come off as particularly academic in tone, it would in fact be of more interest to specialists in my opinion.