Death's door - James R. Benn

Lieutenant Billy Boyle could have used a rest after his last case. But when his girlfriend Diana Seaton - a British spy - goes missing in the Vatican where she was working undercover, he insists on being assigned to a murder investigation there so he can try to help her. An American monsignor is found murdered at the foot of Death's Door, at Saint Peter's Basilica. Just a short walk away, Diana is being held in an infamous prison. Can he dare a rescue, or will a failed attempt alert the Germans to his mission and risk an open violation of Vatican neutrality?