Eclipse of the heart

From Goodreads:

Zachary Grayson, a seemingly passionless middle-aged San Franciscan who makes more than a comfortable living writing trendy cookbooks, is asked a curious question during the idle chatter of a dinner party: What would he kill for? The question seems so preposterous, Grayson doesn't bother answering. His life is quiet, comfortable. Other than wandering through the city's bookstores, catching a foreign film at the Lumiere, and having an occasional dinner alone at the Elite Cafe, he lives an adventureless, almost monastic existence. But when a stranger enters his life, everything is turned upside down, and a supposedly harmless holiday in Puerto Vallarta becomes a life-changing odyssey, an adventure punctuated with love, humor, deceit, and murder.