Following up a first novel with a solid second book is a challenge and Rowling steps up to the plate. She uses a clever mixture of the familiar, starting in Primrose Lane, and the new - playing with the expected routine to tantalise the reader. The characters are well established but allowed to grow as they get older.
Harry Potter is a pastiche of genres in some ways and the 'children as detectives' is a tried and true plot device that Rowling exploits to full advantage in her series. In this second book there are also darker elements that begin to intrude giving a hint at the deeper themes that Rowling explores as her series progresses.