Mark--The Callahan Brothers

They are the Bad Boys of Brazos Bend—Matthew, Mark, Luke, and John Callahan.
New York Times, USA Today, and Publishers Weekly bestselling author Emily March invites readers to visit Brazos Bend, the small hometown of the Callahan brothers snuggled deep in the heart of Texas.
Members of Mark Callahan's old military unit are being murdered. To discover who's responsible and why the former intelligence officer must partner with the only team member more dangerous than he is...Annabelle Monroe, his ex-wife.
Now a private investigator, Annabelle would rather trade her sidearm for a slingshot than work with Mark again. But a shattered heart is no excuse to avoid a reunion when lives are at stake.
Together, she and Mark go hunting a killer. Long days and steamy nights make it impossible to deny the desire still burning between them, but the reasons for their messy breakup haven't changed. The bravest man she has ever known is still afraid to risk his heart.
But when the killer's trail leads to Brazos Bend and everything Mark holds dear is at stake, will he finally take the leap? Or will it be too late?