The MIdnight before Christmas by Wm. Barnhardt

 This is a stand-alone book by Barnhardt, set on Christmas Eve in Oklahoma City. Megan is a lawyer stuck working over the holidays. A frantic woman comes into her office begging for help getting a restraining order against her ex-husband. She says he's a violent man who will kill their young son if they can't be together. Megan gets her the restraining order but then begins to think there might be something fishy about the woman's story and starts doing a little investigating on her own, and after she talks to the ex-husband, she is convinced she is right. Lots of good twists and turns and a kind of entertaining subplot about Christmas presents that keep getting passed along.