Now and Then by Robert B. Parker

Now and Then (Spenser) - Dennis Doherty asks Spenser to investigate his wife’s abnormal behavior.  Spenser does and finds her not only cheating, but she is relating information her husband has given her to her lover, Perry Alderson.  Shortly afterwards, both Dennis and his wife are murdered.  Perry Alderson attempts to retrieve the tapes from Spenser by 1^st abducting Susan, then when that fails, by wooing her away from Spenser.