On a Pale Horse: Incarnations of Immortality #3 (1992)

Death Rides A Pale Horse! Magic has always existed side-by-side with science, but in its current sophistication has changed the world much in the past century. Now Death is a real being, a job served by a man names Zane who accidentally murdered "Death" who served before him. He is but one of the Incarnations of Immoratility, along with Fate, Time, War, and mother Nature. Book Three: In "Death's Dominion," as Zane becomes further integrated into his new job, he also becomes more involved with beautiful Luna Kaftan -- who reveals a past that threatens her future and thickens the mystery surrounding a plot to manipulate the office of Death!