Dark corners : a novel / Ruth Rendell.

Enhanced descriptions from Syndetics:

"A spectacularly creepy and macabre tale" ( Entertainment Weekly ) of blackmail, murders both accidental and opportunistic, and of one life's fateful unraveling--from Ruth Rendell, "one of the most remarkable novelists of her generation" ( People ), writing at her most mesmerizing.

When his father dies, Carl Martin inherits a house in an increasingly rich and trendy London neighborhood. Carl needs cash, however, so he rents the upstairs room and kitchen to the first person he interviews, Dermot McKinnon. That was colossal mistake number one. Mistake number two was keeping his father's bizarre collection of homeopathic "cures" that he found in the medicine cabinet, including a stash of controversial diet pills. Mistake number three was selling fifty of those diet pills to a friend, who is then found dead.

Dermot seizes a nefarious opportunity and begins to blackmail Carl, refusing to pay rent, and creepily invading Carl's space. Ingeniously weaving together two storylines that finally merge in one shocking turn, Ruth Rendell describes one man's spiral into darkness--and murder--as he falls victim to a diabolical foe he cannot escape.

This is masterful storytelling that gets under your skin, brilliant psychological suspense from Ruth Rendell. "No one surpasses Ruth Rendell when it comes to stories of obsession, instability, and malignant coincidence" (Stephen King).