The first book draw me in. I had to pick up the Chamber of Secrets immediately. And it worked exactly as I hoped it would. The diary concept is genuinely unsettling - a blank book that writes back, the basilisk lurking in the pipes, petrifying students one by one, adds proper menace to what was previously just a fairy tale school.
The mystery structure is tighter here - the attacks, the messages written on walls, the false leads that keep you guessing, all without branching off into unnecessary subplots. The prejudice against Muggle-borns feels natural, uncomfortable, without being forced.