Nigel West is a pen name for Rupert Allason, who has written quite a number of books on espionage. A member of parliament from 1987-1997, he has lectured at both the CIA and, ironically, the KGB.
If you weren't an adult during the years that the Cambridge Five spy ring was seeping into the headlines, this book will send you off to Wikipedia constantly (I should have realized the back cover's statement "everybody knows about Philby, Burgess, Maclean and Blunt - But what about the others?" was a warning.) There is an assumption that you're familiar with the major communist agent cases discovered in British MI5 / MI6 during the 1970s and need minimal references before taking on more.
And there is more. You'll put this book down thinking that Canada, the U.S., and England were veritable sieves of confidential information, while Russia, Czechoslovakia, East Germany, and even South Afrika were able to waltz into "secret" locations and turn previously average citizens into communists at will. The individuals that MI5 and the CIA attempt to use to help their operations generally become irresponsible, while the Russian tools rarely failed. You also get a brief view of the Profumo Affair.
Knowing what we know now, it's really terribly sad, as we receive hints of the levels of tracking the security agencies are able to do (with no real results), while the opposing agencies are able to efficiently run circles around them and recruit naive and idealistic British agents. The book gives an interesting overview - but without details.