I was an admirer of Ruth Rendell and an avid reader of her books from the 1970s.
THE SECRET HOUSE OF DEATH is an early standalone novel published in 1968.
Susan Townsend was the only resident with no interest in the affair going on next door in Orchard Drive, or in the neighbourhood gossip about it. Yet it was Susan who found the bodies of the lovers locked in death. And it was Susan whose own life would be imperilled by a monstrous crime.
From the inside flap of the dustjacket
Louise North doesn't care what the neighbors think. She lets her lover leave his car just outside her house in broad daylight, telling everyone a cockamamie story about him being a central heating salesman. Still, it's a shock when she's found shot dead, covered by the equally dead body of the "salesman." Now Susan Townsend -- the Norths' next-door neighbor, who discovers the bodies -- must help Louise's husband, Bob, get back on his feet. But is she helping a neighbor . . . or a murderer?
The story was filmed for TV in the Ruth Rendell Mysteries 1987