No Rules by Nora Phoenix

Alex is proof that you can be tough as nails…and still be submissive.
Alex loves serving others—when he’s not doing his job as a security specialist. His team is investigating a puzzling case involving a spunky grandma and a dead horse at the racetracks.
Ryan, a grumpy older PI, is all too happy to make use of Alex’s talents, and he seems to delight in pushing Alex’s buttons. Alex is determined not to let Ryan run all over him…even when deep down, he loves it when Ryan gets all dominant and aggressive.
That’s just physical attraction. Chemistry. It doesn’t mean anything.
Ryan has made it clear he wants no entanglements, and Alex can’t ever see himself serving only one man.
Until he can.
Until he wants no one else but Ryan.
Until he’s stupid enough to fall for the one guy who’ll never be able to commit.