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201 pages, Kindle Edition
First published January 1, 1937
"X," answered the inspector. "The unknown quantity, who may upset all our calculations. It's fatal to forget him. Whenever you make a list of possible criminals, you are apt to put yourself in blinkers and forget that anyone exists outside your list. Always put in X, and keep a sharp look-out for him."These words have a powerful effect on Frant, who reveres his boss Mallett.