The Weed that Strings the Hangman's Bag by Alan Bradly
Flavia de Luce didn't intend to investigate another murder - but, then again, Rupert Porson didn't intend to die. When Porson's van breaks down in Bishop's Lacey, he becomes the victim of an electrocution too perfectly planned to be an accident.
Putting down her sister-punishing chemistry experiments and picking up Gladys, her bicycle, Flavia uncovers long-buried secrets in Bishop's Lacey, a seemingly idyllic town that nevertheless is a home to a mad woman living in its woods, a prisoner-of-war with a soft spot for the English countryside, and two childless parents with a devastating secret. It's possible that Rupert Porson's van didn't break down so accidentally in this charming hamlet. And it's possible that his killer may lure in Flavia way over her eleven-year-old head, to a startling discovery about the chemical composition of vengeance.