The Cheyne Mystery by Freeman Wills Crofts

When Maxwell Cheyne's paroxysm of fury diminished and he began once more to think collectedly about the unpleasant situation in which he found himself, a startling idea occurred to him. Here at last, surely, was the explanation of his previous adventures! The drugging in the hotel in Plymouth, the burglary at Warren Lodge, and now his kidnapping on the Enid were all part and parcel of the same scheme. It was for Price's letter that his pocketbook was investigated while he lay asleep in the private room at the Edgecombe; it was for Price's letter that his safe was broken open and his house searched by other members of the conspiracy, and it was for Price's letter that he now lay, a prisoner aboard this infernal launch.

A valuable document, this of Price's must surely be, if it was worth such pains to acquire! Cheyne wondered how it had never occurred to him that it might represent the motive of the earlier crimes, but he soon realized that he had never thought of it as being of interest to anyone other than Price. Indeed, Price himself referred to his enclosure as "some private papers, of interest to myself only. " In that last phrase Price had evidently been wrong, and Cheyne wondered whether he had been genuinely mistaken, or whether he had from distrust of himself deliberately misstated the case in order to minimize the value of the document. Price had certainly not shown himself anxious to regain it at the earliest possible moment. On the conclusion of peace he had not accepted demobilization. He had applied for and obtained a transfer to the Middle East, where he had commanded one of the transports plying between Basrah and Bombay in connection with the Mesopotamian campaign. So far as Cheyne knew, he was still there. He hadn't heard of him for many months, not, indeed, since he went out.