At Her Majesty’s Secret Service: The Chiefs of Britain’s Intelligence Service, MI6 by Nigel West Frontline Books, 2016, 246 pages, $50 Spymaster: The Life of Britain’s Most Decorated Cold War Spy and Head of MI6, Sir Maurice Oldfield by Martin Pearce Bantam, 2016, 391 pages, $50 ____________________________________   Intelligence is about people and the study of people.                                                     —Maurice Oldfield   Nigel West has had a long-standing fascination with espionage and has published numerous books on the subject. Although some of the material in At Her Majesty’s Secret Service has also been covered in earlier books, much of it is…