Code of Valor, Blue Justice Series by Lynette Eason and published by Revell.
What Detective Brady St. John really needs is a relaxing vacation. Unfortunately, just as the sun is setting on his second day at a friend's cabin on Lake Henley, he hears a scream and races to rescue a woman from her would-be killer. When the killer escapes only to return to finish the job, Brady vows to utilize all of his many resources to keep her safe and catch those who would see her dead. Facts about why she was being attacked, there are some things you just don't share with a stranger. Little does she suspect that the secret she is keeping just might get them both killed.