The Middle of Things by J.S. Fletcher

Unknown to those who had taken part in the conference at Viner's house, unknown even to Carless, who in the multiplicity of his engagements, had forgotten the instructions which he had given on the previous afternoon to Portlethwaite, a strict watch was being kept on the man around whom all the events of that morning had centred. Portlethwaite, after Methley and his client had left Carless and Driver's office, had given certain instructions to one of his fellow-clerks, a man named Millwaters, in whose prowess as a spy he had unlimited belief. Millwaters was a fellow of experience. He possessed all the qualities of a sleuth-hound and was not easily baffled in difficult adventures. In his time he had watched erring husbands and doubtful wives; he had followed more than one high-placed wrong-doer running away from the consequences of forgery or embezzlement; he had conducted secret investigations into the behaviour of persons about whom his employers wanted to know something. In person and appearance he was eminently fitted for his job -- a little, inconspicuous, plain-featured man who contrived to look as if he never saw anything. And to him, knowing that he was to be thoroughly depended upon, Portlethwaite had given precise orders.

"You'll go up to Lancaster Gate tonight, Millwaters, and get a good look at that chap," Portlethwaite had told him. "Take plenty of money -- I'll speak to the cashier about that -- and be prepared for anything, even to following, if he bolts. Once you've seen him, you're not to lose sight of him; make sure of him last thing today and first thing tomorrow. Follow him wherever he goes, make a note of wherever he goes, and particularly of whoever he meets. And if there's need, ring me up here, and let's know what's happening, or if you want assistance."