Crossfire. The Guardians, #1

He's a former Navy SEAL turned private security operator - dangerous, relentless, and completely off-limits. She's a brilliant translator running a fragile new business while caring for her dying father. Sam Reston has wanted Nicole Pearce from the moment he laid eyes on her, but between undercover operations and her ice-cold walls, getting close has been mission impossible. Until it isn't. When a deadly threat reaches Nicole's door, Sam steps out of the shadows and into her life.
But keeping her safe means keeping her close, no matter the consequences. Nicole knows better than to trust a man built like a weapon who moves like a ghost. Sam knows better than to let a woman become an obsession.