There is a Muggle Protection Act that prevents people of the magical community from using magic in a way that Muggles can see or come in contatct with. Each book charts a year's progress through Hogwarts school for Harry and his friends. Harry Potter and the Chamber of Secrets followed in 1998, featuring Harry, Hermione and Ron in their second year at Hogwarts. They are immersed in their daily round of lessons when Harry hears evil voices. Sinister messages appear on the wall.
Nothing prepares the three friends for what is about to happen.
Harry Potter and the Prisoner of Azakaban emerged in 1999. The swotty Hermione is based on Rowling herself as a schoolgirl. Ron is based on her oldest friend, Sean. Harry is partly imaginary but Rowling says there is a lot of herself in him. Aunt Marge was based on her maternal grandmother, Frieda.