Champagne for One — Rex Stout

Faith Usher had a decidedly morbid personality. She talked about taking her life, and kept cyanide in her purse. So when she collapses and dies from a lethal champagne cocktail in the middle of a high society dinner party, everyone calls it suicide--including the police. But Archie was watching it all, and suspects it was murder. So does Nero Wolfe, especially after he's warned by four men against taking the case. For the world's most formidable detective it is a tantalizing puzzle involving an unlikely combination of philanthropy, deception, blackmail, and an unrepentant killer who just may have committed the perfect crime.