This book was written in part to as a response to The Master Spy by Phillip Knightley. Costello claims Knightley went easy on Kim Philby.
Costello has a much more critical view of all the Cambridge spies than did Knightley. Almost all of Costello’s conclusions are stated as generous for their modesty and the reality was probably much more serious damage to intelligence services but covered up by the Intelligence services themselves, English government, as well as the crown.
This is a long book and could have been shorter but I enjoyed reading all of it, depressing as it was. Costello covers all of the spies but focuses on Anthony Blunt's career. Costello argues that Guy Liddell was probably the “grandfather spy” to all the rings operating out of British Intelligence agencies. Many good factual items in this book are scattered throughout. Mask of Treachery is an excellent source of information on methodology, sources, and tactics of WWII and Cold War espionage. The author is himself a Cambridge educated historian. ( )