Spenser 31 Bad Business by Robert B. Parker

When Marlene Cowley hires Spenser to see if her husband Trent is cheating on her, he encounters more than he bargained for. Not only does he find a two-timing husband, but a second investigator as well, hired by the husband to look after his wife. As a result of their joint efforts, Spenser soon finds himself investigating both individual depravity and corporate corruption. It seems the folks in the Cowley's circle have become enamoured of radio talk-show host Darrin O'Mara, whose views on courtly love are clouding some already fuzzy minds with the notion of cross-connubial relationships. O'Mara's brand of sex therapy is unconventional at best, unlawful - and deadly - at worst.