Confessor by John Gardner

When Gus Keene, one of the best interrogation artists or "confessors" the British Secret Intelligence Service has ever known, is blown to bits in a car explosion, the service calls his friend and former associate "Big Herbie" Kruger out of retirement to investigate. As Kruger delves into his friend's past and the black arts of a "confessor", he discovers a strange fact--the victim was not only a master at uncovering the truth, but of concealing it as well.