A Fine and Private Place — Ellery Queen

Ellery Queen investigates a mobster whose bizarre death is marked by the number 9

Nino Importuna has a soft face, but when he smiles, it's terrifying. His Central Park penthouse is lavish, but it was bought with the blood of his enemies. His criminal empire controls mining, electronics, and food -- legitimate corporations that he runs with a murderer's touch. When he catches one of his capos stealing from him, Importuna could either kill the man or send him to prison. Instead, he makes a simple demand: He wants the thief's daughter to be his wife.
 
On their 5th wedding anniversary, Importuna signs his fortune over to his young bride. Soon after, the 9-fingered mobster is killed by 9 blows to the head and Ellery Queen receives a 9-letter note that holds the key to the homicide. In the legendary detective's final case, 9 is the magic number.