The Shadow of the Dead Hand by Fred M. White

Peter Lantary had not meant precisely what he said when he told Inspector Dwight that the man Martin was the key to the situation or that, at any rate, he held the key to it. He did not doubt for a moment that Martin was one of the prime characters in the criminal cast, but there was one individual, at any rate, who could have told the police a great deal more. Naturally enough, Lantary wanted to get to the bottom of the business, but he wanted to do it in his own way, and show Scotland Yard that there were other people outside the organisation who could handle a mystery just as well as those who had been trained in the tracking down of professional criminals. Also, he had been just a little piqued by the way in which Dwight had treated him in the first instance. The inspector had, first of all, been inclined to smile at Peter's suggestion that the dead man was not Murdstone at all. And when he had, at length, been convinced that such was the fact, he had not gone out of his way to congratulate his amateur colleague. And Peter, with his sense of humour, and his persistent leg-pulling, was not likely to take Scotland Yard into his confidence until he was ready for the grand coup, which he began to see dimly shaping itself before him.

All the same, he wanted Martin carefully watched, and he saw no reason why Scotland Yard should not do what he called the donkey work, whilst he himself was engaged on the finer points of the problem. And the real critical point lay somewhere between Murdstone and the woman called Countess Visconti.