Harry Potter and the Goblet of Fire

Harry gets pulled into the Triwizard Tournament in book four even though the competition is supposed to be restricted to wizards over seventeen, and the Goblet of Fire's selection at Hallowe'en changes the shape of his whole year at Hogwarts. The tournament brings two other wizarding schools, Beauxbatons and Durmstrang, into the story for the first time, built around three dangerous tasks that test the champions' skill and nerve — including a task in a dragon's lair and a rescue from the depths of the Black Lake. It runs 757 pages, and Pottermore's catalog lists it for ages 6 to 10, though nothing about dragons and a returning Dark Lord reads like content for a six-year-old.

This is the book where the series visibly changes gears: the school-year adventure structure is still there, but it's carrying real threat now, and Harry, Ron, and Hermione deal with first crushes and shifting friendships on top of a competition actively trying to get people killed. The tonal shift is intentional and worth knowing about going in, since the emotional intensity and darker turns are a real step past the first three books. Best handed to a reader who's already worked through the earlier Harry Potter books and is ready for the series to stop being purely comfortable.