The Cambridge Five, as you probably know, was a ring of spies all recruited by the Soviets after having become communists during their years at university in the thirties. Four of the five--Kim Philby, Donald Duart Maclean, Guy Burgess, and Anthony Blunt--have been definitively known since the fifties and early sixties. The presence of a fifth member of the group was long suspected, and many consider John Cairncross to be the likeliest candidate.
Charles Cumming, in his spy thriller The Trinity Six takes the premise a step further by positing a sixth spy, one who was never caught (or defected), and who may still be alive. His novel slants the action differently than most spy novels by making the protagonist not a spy or an intelligence officer, but rather a professor of Russian studies. Sam Gaddis is the fortyish academic, divorced, behind in his mortgage and tax payments and being pressed by his ex-wife for additional child support. When asked by a journalist friend to co-write a book based on interviews she's currently conducting about the possible sixth man, Gaddis jumps at the chance. Days later his friend is dead. As he pursues the leads she had begun to uncover, Gaddis discovers most people unwilling to discuss the subject with him...and those who do seem to end up dead as well.
Cummings has written a tidy (though perhaps a bit coincidence-ridden) thriller which flies along at a satsifyingly brisk clip. Sam Gaddis is mopey and self-centered, but smart and capable as well. The Trinity Six, while not one for the ages, is still a worthy contribution to the spy thriller genre, and well worth a read.