Pieces of Her by Karin Slaughter

On her 30th birthday, Andrea Oliver celebrates the occasion at a local diner with her mother, Laura. Moments later, she witnesses the unthinkable on a day that would forever change her life. After her mother attempts to stop a shooter from killing more victims, Andrea notices that Laura has flipped the script and watches her violently dispose of the threat. Visibly shaken, Andrea is left with wavering thoughts and conflicted feelings about her mother's actions. Soon after, video footage of the incident at the diner becomes a viral hit, exposing Laura's identity to her past enemies. As Andrea begins to gather the pieces of Laura's past on her journey to uncover the real identity that her mother has forsaken long ago, what other truths will she find?