Player on the Other Side — Ellery Queen

On the sprawling estate of a bizarre wealthy family, a series of cryptic notes brings deadly regards in this classic from a legendary mystery author.

York Square is a tidy private garden surrounded by four matching castles, each inhabited by a different branch of the York family. There's Robert, commanding and icy; Myra, gentle and ill; Emily, who would prefer to live in a cottage; and Percival, who has many personal secrets. Watching them all is the gardener, Walt, who sees more than any of them realize. When an anonymous scribe starts sending him letters of praise, Walt is happier than he's ever been. But when a strange card marked with the letter J heralds the death of Robert, the happy garden begins to wilt.
 
Unlocking the puzzle of the bizarre notes falls to the legendary Ellery Queen. He finds that the Yorks are locked in a ghoulish bargain -- one that can only be escaped by death.