Love interest nearly always weakens a mystery because it introduces a type of suspense that is antagonistic to the detective's struggle to solve a problem.

When I split an infinitive, God damn it, I split it so it stays split.

I said something which gave you to think I hated cats. But gad, sir, I am one of the most fanatical cat lovers in the business. If you hate them, I may learn to hate you. If your allergies hate them, I will tolerate the situation to the best of my ability.

The streets were dark with something more then night.

At least half the mystery novels published violate the law that the solution, once revealed, must seem to be inevitable.

Police business is a hell of a problem. It's a good deal like politics. It asks for the highest type of men, and there's nothing in it to attract the highest type of men.

A good title is the title of a successful book.