Sinister Heights — Loren D. Estleman

Detroit PI Amos Walker steps into a lethal family feud when the beautiful widow of a powerful industrialist hires him to find her late husband's illegitimate children

Leland Stutch was building automobiles before Henry Ford ever dreamed up the Model T. He dominated Detroit for most of the 20th century as the auto industry soared and then began its long, slow descent. When Stutch's widow contacts Amos Walker, the private eye expects to meet a doddering old lady. Instead he encounters Rayellen, a 30-something beauty with washboard abs and 1 of the most unusual propositions he's ever heard.
 
Unconcerned with matrimonial vows, the most powerful man in Detroit left mistresses -- and love children -- all over Michigan. To stave off any future paternity suits, Rayellen hires Walker to locate Stutch's illegitimate offspring and pay them off -- a seemingly simple task that draws the detective into a dysfunctional family's war zone and a violent case of kidnapping and murder.
 
Sinister Heights is the 15th book in the Amos Walker Mysteries, but you may enjoy reading the series in any order.