The Castle Series is the name of the series of books that follow the lives of Howl Jenkins Pendragon, Sophie Hatter, their Fire Demon Calcifer, and their Moving Castle. All of the books are written by Dianne Wynn Jones.
Howl's Moving Castle is the first novel to be written in the series. Published in 1986, it follows the story of a young woman who leaves her life of near servitude in her mother's hat shop after she has been cursed to look like an old woman, and ends up living with a very powerful and mysterious sorcerer.
Howl's Moving Castle won a Boston Globe-Horn Book Award in 1987, and became an ALA Notable Book for both Children and Young Adults. In 2004, it was loosely adapted into an animated movie by Japanese director Hayao Miyazaki, which was nominated for an Academy Award for Best Animated Feature.
Castle in the Air is the first sequel to Howl's Moving Castle, and second book in the Castle Series. Published in 1990, it is often debated whether or not it counts as an actual sequel to Howl's Moving Castle. Though set mainly in the land of Ingary and deals with several prominent characters of the first book, those characters are only revealed late in the novel, and play a much more minor role than the novel's primary protagonists. Nevertheless, the book is officially labeled as the "Sequel to Howl's Moving Castle."
House of Many Ways is the most recent of the Castle Series to be written and published, and is the second sequel to Howl's Moving Castle. Though, like the last book, still told through the eyes of an outside character, this story deals more directly with the characters of the first novel, so is often considered to be its true sequel. Published in 2008, it was long awaited by fans of the series - a number which had greatly grown after the release of Miyazaki's movie.