The Cat and the Canary by John Willard

Murder... Mystery... Mayhem! The family of a rich eccentric attends a reading of his will at midnight 20 years after his death. If his sole heir should be insane, a second will is to be opened naming a different heir. Everyone among the possible heirs is suspected, but not until the final second is the extraordinary mystery solved. “They have watched my wealth as if they were cats, and I -- a canary.”