Harry Potter and the Chamber of Secrets
JK Rowling
** spoiler alert **
Harry Potter and the Chamber of Secrets is my favorite of the Harry Potter books. That's mostly because of the long final scene with the Basilisk and Tom Riddle's diary. I remember still, more than 20 years after first reading it, how surprised I was. It was one of those, "No, she's wouldn't dare go there!" moments. She did dare, and she did go there. Ginni Weasley drained and near death, Harry himself poisoned by the basilisk's venom. And, of course, the pièce de résistance -- Tom Riddle appearing in corporeal form from his diary, apparently invulnerable, until Harry thinks to stab him with the Basilisk's fang.
It was intense.
It was the moment when the Harry Potter books stopped just being fun and became serious.
Of course, it foreshadowed much that was to come. We later learn that the diary was one of Voldemort's horcruxes, perhaps the first. Destroying horcruxes is the main job of the last books in the series, but feels less intense there -- more of a sort of assembly-line process. Handsome, clever, charming Tom Riddle was a surprise, a new, more intimate view of Voldemort. And Ginni, whom Harry rescues here, eventually becomes his love and his wife. I don't think anyone actually dies in Chamber of Secrets, but you begin to feel that that is a real possibility, and of course it does eventually happen.