Bad Business by Robert B. Parker

A cheating husband and a wayward wife provide Spenser with an unconventional and dangerous surveillance job. 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.