Only Hillary Clinton could make this book boring. “What Happened” (Simon & Schuster, 494 pages, $30) recounts the events of her 2016 presidential campaign, defends her own conduct and decisions, and assigns the blame for her loss to Donald Trump in the November election. Ms. Clinton has always been a tough critic of her foes. Surely we were in for a scorcher. And yet it’s a snoozer—excruciatingly tedious.

Readers of her first two memoirs won’t be surprised. The reason for the dreariness of “Living History” (2003), about...