British readers' appetite for fresh slants on Soviet penetration of UK security services is apparently insatiable. Here, military historian West (The Sigint Secrets, A Thread of Deceit, etc.) offers yet another overview of ongoing efforts by MI5 (England's equivalent of the FBI) to identify turncoats who have betrayed state secrets to the Kremlin since WW II. A best-seller in Great Britain, his gossipy, often bitchy, but evenhanded overview rebuts the fifth-man case first made against Roger Hollis (director-general during the 195665 period) by Chapman Pincher in Their Trade is Treachery (1981) and more recently in Peter Wright's Spycatcher. On the basis of persuasive if largely inferential evidence, West names Graham Mitchell (Hollis' deputy until his precipitate retirement in 1963) as the probable traitor. In reaching his revisionist conclusion, the author reviews the still murky circumstances surrounding the exposure of the four members of the so-called Cambridge Comintern--Guy Burgess, Donald Maclean, H.A.R. (Kim) Philby, and Anthony Blunt. That vital MI5 plans and operations remained an open book in Moscow long after their detection convinced counterintelligence agents that a Judas was still at large in the upper echelons of the service. While mole-hunters launched a series of wide-ranging inquiries (frequently prompted by defectors), they were never able to secure proof enough to bring formal charges against a high-ranking fifth man. In addition, investigators were effectively barred from following up on some promising leads by the establishment's propensity for protecting the old-boy network. In rejecting a probe of the possibility that Oleg Penkovsky (a GRU colonel who had kept Anglo-American agencies supplied with apparently first-rate information during the early 1960's) was a KGB plant, for example, West reports that one high-level hawkshaw remarked: ""Too many knighthoods depend on this one."" While of an open mind on Penkovsky, the author is convinced and convincing about Mitchell's culpability. Beyond isolating his prime suspect by a painstaking process of elimination, he offers a close reading of an error-riddled White Paper drafted by Mitchell, which muddied the waters in the wake of the flight by Burgess and Maclean. Overall, then, fine fare for students of the Cold War's high-stakes chess games. West's literate and consistently absorbing narrative has eight pages of photographs (not seen), plus appendices, including the full text of Graham Mitchell's 1955 White Paper--with annotations.