Bad Business by Robert B. Parker

Bad Business (Spenser) - Marlene Cowley hires Spenser to find out if her husband, Trent, is cheating on her, and she wants pictures.  While following the husband, Spenser meets two other private detectives, one following Trent, the other following his lover.   As he is detecting, Spenser is faced with a murder, a sex talk show host who preaches free love, and an offer of a high paying low effort position out of state.