Josephine Tey

Grant called in Williams, and together they considered the plain, cheap, white envelope and the strong, plain capitals.
"Well," said Grant, "what do you know?"
"A man," said Williams. "Not well off. Not used to writing much. Clean. Smokes. Depressed."
"Excellent!" said Grant. "You're no good as a Watson, Williams. You get away with all the kudos."