The Mother Hunt by Rex Stout

When an abandoned baby is left on her doorstep, the young socialite widow knows only too well the identity of the father: her deceased philanderer of a husband. But who is the mother? The case seems like child's play to Wolfe, until the first dead body. While the police nurse their grudges against him, and the widow nurses Archie, the genius sleuth and his sidekick look for the hand that rocked the cradle. But nothing can pacify the killer, who's found the formula for murder—and is determined to milk it for all it's worth…