Her Husband's Army Buddy. McLeod Security, #1

There should only ever be two people in a marriage... Sandy McLeod has been perfectly happy in her D/s relationship with her husband Zane for the last ten years. Until his old army buddy re-enters their life. Sean Manson is altogether too handsome, too virile, far too much of anything. The man oozes dominance, danger, and leashed aggression, and Sandy can't help but respond to him. Neither can Zane.
He walked away from Sean once. Now, he's back, long suppressed feelings bubble to the surface and cannot be denied. When Sandy agrees to a threesome, happiness seems within their grasp. Sean's demons, however, threaten to destroy everything they hold dear. Sean never meant to come between husband and wife, let alone hurt either one of them. Surely, the only solution is to walk away from them both.
When you're the missing piece, however, walking is simply not an option.