Big Little Truths by Liane Moriarty

It has been a decade since the ill-fated quiz night at Pirriwee High, where a man died and accusations of murder ran wild. For mothers and best friends Madeline, Celeste and Jane, the fallout still feels fresh. Their children are now moody teenagers, and life remains as demanding as ever. Madeline is busy planning her 50th birthday, unaware she's about to be upstaged. Jane is content in her marriage to Tom - perhaps too content to notice its cracks. Celeste, still haunted by her husband's death, fears what her formidable mother-in-law might do next. When the school receives an anonymous package containing a severed finger, amid reports of a suspicious man near students, suspicion spreads quickly. Is it a prank, a threat, or something far darker? As secrets surface, the truth proves as dangerous as ever.