Stafford, D: Spies Beneath Berlin

Operation Stopwatch/Gold, said CIA chief Alan Dulles, was one of the most valuable and daring projects ever undertaken. In 1955 it ran a tunnel 800 metres under the Russian sector of Cold War Berlin, and for more than a year tuned into Red Army intelligence. This was an almost impossible trick: apart from the technical wizardry needed, any noise or vibration could have given the game away.
That the operation succeeded is even more surprising than it looks. Trust, even between allies, was dangerous. Despite the Burgess and Maclean affair, the Americans had decided that co-operation was safe once more, and Stopwatch/Gold was a joint CIA/MI6 project using British expertise from a prototype in Vienna. This was a mistake: there was another mole in the British secret services, and the KGB knew about the tunnel even before it was built.
This book tells the story. David Stafford draws on eyewitness interviews and the full range of sources. Ironically, it was the Russians who supplied the minutes of the meeting that OK'd the tunnel. They had been taken by George Blake (who was of course the mole).