Gentlemen of Crime by Arthur Gask

"Yes," said the professor, frowning, "I received Fenton's letter this afternoon, just before three. He put the arranged signal in his window early in the morning, a blind drawn unevenly this time, and Mattin sent a boy up to the castle to collect a pair of boots for mending. The letter came out under the cork sole of one of the boots, and Mattin bicycled with it here. Now I'll read it to you."

He read the letter slowly and without comment, and then, leaning back in his chair, regarded his two companions silently.