Sudden Mischief by Robert B. Parker

When Susan's ex-husband, Brad, appears after a decades-long absence, nearly broke and the object of a sexual-harassment suit, Spenser reluctantly agrees to help. As he investigates the circumstances surrounding the suit, he discovers that fund-raiser Brad is swimming in very deep water: mobsters, who were using his fund-raising campaigns to launder money, have discovered he was cooking the already cooked books and aren't at all pleased. The deeper Spenser digs, the more bodies he uncovers and the more culpable Brad appears to be.