The Quaking Widow, by Robert Colby (epub/Kindle/pdf)

Burt Keating had been trying to make the acquaintance of his pretty red-headed neighbor but he wasn’t getting very far until the day she escaped from a strange roadside encounter. For when it turned out that murder had resulted from the affair, she threw herself hysterically into Burt's arms, pleading for his help.

It seems she had inherited a sealed box, whose contents were entirely unknown. Certain parties wanted that box badly. They had offered her a fortune -- or sudden death.

Burt knew then that chivalry can go too far, for there was no longer any way out once he’d tangled with THE QUAKING WIDOW.