The Valley of Ghosts by Edgar Wallace

Scottie's installation at the Nelsons' served a double purpose. It recruited to Andy a shrewd, if unprincipled, lieutenant, and it gave him a more comfortable feeling about leaving Stella with no other protection than her father. There was the indisputable fact that somewhere at large was the murderer, and that that murderer had possibly seen the interview between the girl and Merrivan, and might, to save his own skin, implicate her. For how had Stella's scarf reached the orchard? For what purpose it had been taken he could not guess, but this was clear--that the murderer was well aware of her presence. He drove to town that morning, carrying with him the charred diary which had told him nothing, for half the pages had been torn out and burnt separately before the book was thrown upon the fire. His first call was at Ashlar Buildings.

Mr Abraham Selim's office was in the hands of the police, and the predecessor of Sweeny had been found and was waiting for him. The most important find that the police had made was a letter which was addressed to Sweeny. It was evidently in answer to one which Sweeny had left for his master, and it dealt with the prosaic matter of office-cleaning and the expenses to be incurred. Its importance, from Andy's point of view, was that the writing was identical with that which had been discovered on Merrivan's desk.