By J.K. Rowling
The one where something is turning students to stone and a whispering voice moves through the walls, the book that shows your child a series can get bigger and scarier at the same time.
Harry is back for his second year at Hogwarts, and something is very wrong. A message appears on a corridor wall in dripping red letters: the Chamber of Secrets has been opened, enemies of the heir beware. One by one, students are found petrified, frozen solid as statues, and no one knows what is doing it. Harry alone can hear a cold voice hissing through the pipes and the walls, and the whole school starts to wonder if he is the one behind it. With Ron and Hermione he chases the mystery through a haunted bathroom, a giant spider's lair in the forest, and finally down into the Chamber itself, where a monster and a memory of the young dark wizard Voldemort are waiting.
Your child tore through the first book, and this is the perfect next rung: the same reading level, a little longer, and shaped as a real mystery with clues to gather and a culprit to guess. It reads at the same comfortable pace as book one, so the extra pages never feel like a wall, and the payoff of solving the puzzle keeps a reader turning them. It is also where the series signals that it is going to grow, the danger is a notch sharper here, which is exactly what a reader who is ready to climb wants next. And beyond it the books keep getting thicker, so the stamina your child builds here has somewhere to go.