The Clue by Carolyn Wells
On the eve of her wedding, stunning heiress Madeleine Van Norman is found stabbed to death in the library of her palatial country mansion, killed by a single thrust from her Venetian letter opener. Suspicion falls by turns to the groom who loved another, the cousin who stands to inherit her fortune, the woman the groom loves, the murdered woman's secretary, and the former lover of the murdered woman's uncle who will inherit the mansion. A suicide note is found next to her body, but the evidence points to murder. The house had been securely locked with no sign of a break in. The local authorities are mystified. With pressure mounting, they call on the services of the famed detective Fleming Stone to resolve the case. In the end, his solution rests on a single tiny clue.