One of the things I forgot about Chamber of Secrets, in my cynical post-literature-degree hipsterishness, is just how menacing it is. I defy you to read Enemies of the heir, beware without even a tremor running up your spine.
It's another tightly-plotted and suspenseful storyline, filled with bits and pieces of Hogwarts life which are interesting in themselves as well as all falling together at the end to create a very satisfying denouement - something which is surprisingly rare. Rowling's good at planting red herrings, too, and making sure that our readerly expectations are subverted without frustrating them so much that we get annoyed and throw the book away in disgust (also surprisingly rare).
She is, in short, good (very good) at writing exactly this kind of story, a lowish-stakes magical mystery with just the right amount of menace, plenty of humour and a properly immersive world (and I expect there's an interesting essay to be written about the way Riddle's diary feeds off Ginny's teenage emotions). Which makes it all the more puzzling that, after book three, Prisoner of Azkaban, she moved away from this compact and entertaining format into ever more padded and ever less interesting stories about teenage angst and nothing much else. They lose a sense of wonder, I think, a sense of legends beyond our knowledge, as everything becomes dissolved down into a kind of soup of Horcruxes and Hallows and rivalries and war. Re-reading these earlier books reminds me what the Harry Potter books could have been, rather than what they eventually became.