Millionaire Nino Importuna had a lifelong obsession with the number nine - so it was only fitting that he should be murdered on the ninth day of the ninth month, and that his watch stopped at nine minuten past nine.
Investigating the murder, Inspector Queen and his son Ellery receive nine false clues - but what about the tenth? Did Nino's beautiful young widow and her secret lover really batter him to death? And who is the mysterious Mr.E?
This fascninating mystery of love, greed and death is vintage Ellery Queen.

The 9-word clue was one of 9 cryptic notes that had been sent to taunt Inspector Queen and Ellery 9 days after the murder. Nino Importuna had been obsessed with the number. He had lived by it. Now the killer who brought a trio of gory deaths to Nino's 9th-floor penthouse at No. 99 East was camouflaging his identity in a jungle of 9s. And daring Ellery to find him. The case was destined to be a dazzling contest of wits--to the 9th degree!

Nino got his jollies. Cat of Nine Tails whippings

The last EQ novel is OK but not too much more. EQ's first solution is so patently wrong as to be worthless as a red herring, and the "right" solution is not that hard to figure out either.