The Secret Mark — Roy J. Snell

Lucile Tucker's slim, tapered fingers trembled slightly as she rested them against a steel-framed bookcase. She had paused to steady her shaken nerves, to collect her wits, to determine what her next move should be. “Who can it be?” her madly thumping heart kept asking her. And, indeed, who, besides herself, could be in the book stacks at this hour of the night?