Harry Potter and the Order of the Phoenix by J. K. Rowling

Harry Potter and the Order of the Phoenix is the fifth in the Harry Potter series written by J. K. Rowling, and was published on 21 June 2003 by Bloomsbury in the United Kingdom, Scholastic in the United States, and Raincoast in Canada. Five million copies were sold in the first 24 hours of publication.
The novel features Harry Potter's struggles through his fifth year at Hogwarts School of Witchcraft and Wizardry, including the surreptitious return of the antagonist Lord Voldemort, O.W.L. exams, and an obstructive Ministry of Magic.
Harry Potter is spending another horrible summer with his dreadful Aunt Petunia and Uncle Vernon when a group of evil spirits called Dementors stage an unexpected attack on Harry and his cousin Dudley. After he uses magic to defend himself and Dudley, an act that later gets Harry Potter expelled from Hogwarts, he is visited by a group of wizards and Mad-Eye Moody and the is whisked off to number 12, Grimmauld Place, London. Number 12 is the home of Harry Potter's godfather, Sirius Black, and the headquarters of the Order of the Phoenix. The Order is a group of witches and wizards, led by Hogwarts headmaster Albus Dumbledore, dedicated to fighting the evil Lord Voldemort and his followers. The Order is forced to operate in secrecy, outside of the jurisdiction of the Ministry of Magic, which is headed by the dense and corrupt Cornelius Fudge. Fudge refuses to believe that Lord Voldemort has returned ... (Summary by Wikipedia)