Kneading the Kink: A Hotwife Story by Matthew Lee

Happily married Carlie Jo Chatsworth has thrown her back out and the golf tournament is only a few weeks away. The somewhat prudish woman hesitates to visit the cheap health spa near her house for the much-needed physical therapy, but they have the open calendar.
Tony seems nice and is excellent at his craft but there is something sinister about the place. Carlie Jo can't quite put her finger on it until she meets Evan, one of Tony's partners, an ex-stripper and male prostitute with connections to organized crime.
Soon Carlie Jo is making appointments exclusively with Evan, despite her husband's warnings, and the tense situation gets worse.