The Governor's Wife by Michael Harvey
"When Illinois Gov. Raymond Perry is sentenced to prison for fraud and racketeering, he enters an elevator in Chicago's Dirksen Building and disappears. Soon afterward, PI and ex-cop Michael Kelly gets an anonymous email offering him $200,000 to find Perry, no questions asked. Naturally, he begins with Mrs. Perry, an enigma who fears that her husband is dead. Further probing, with help from a cop friend, leads to a mysterious construction conglomerate where Kelly is violently warned off. It appears that a shadowy political cabal is behind a decades-long skimming of road construction funds with the governor so much involved that $60 million has gone missing with him. The ruthless pols are eliminating anyone involved while they pressure Kelly to find the money."
Harvey's books are similar to Michael Connelly and early Robert Parker. I highly recommend any in his series. The only fault I have with the book is that it is too short.