Sinister Heights by Loren D. Estleman
Leland Stutch, creator of an automobile dynasty and master of the town of Iroquois Heights, just outside Detroit, died at an age well over one hundred. Rayellen Stutch, the 30-something widow of Leland, calls on Amos Walker to undertake a most unusual investigation. Since she has more than enough money to see her comfortably through life, she wishes to share the wealth with the various mistresses and illegitimate children Leland had throughout his long, long life. As Walker locates the various individuals and picks up information about Leland's numerous daliances, he learns that a long life of power makes one many, many enemies.