Spying on one’s friends is generally considered a no-no, but some in the British upper class famously elevated it to high art. A Spy Among Friends: Kim Philby and the Great Betrayal delves into the story of arguably the most famous case. “Kim Philby was the most notorious British defector and Soviet mole in history. Agent, double agent, traitor and enigma, he betrayed every secret of Allied operations to the Russians in the early years of the Cold War.