Rough Rider by B. J. Daniels

B.J. Daniels is at the top of her game…the perfect blend of hot romance and thrilling suspense.” – New York Times bestselling author Allison Brennan.
He was sure he could handle one petite PI with an attitude—but he’ll need her help to solve the secret of his missing sister and outwit a ruthless killer…
His family never fully recovered from the kidnapping of his siblings decades ago. Now Boone McGraw finally has a lead on his missing sister’s location, but it means working with feisty private investigator C.J. Knight.
Desperate to solve her partner’s murder, C.J. doesn’t believe her case could possibly be connected to the sexy horse breeder’s investigation…until they find themselves running for their lives.