London, Oct 25 (ANI): Harry Potter is still working as a magic for Bloomsbury publishers. After 20 years of his debut, special editions of the boy wizard's first adventure are boosting the firm's revenue by 15 percent. Bloomsbury released these editions of J.K. Rowling's "Harry Potter and the Philosopher's stone" in June. The covers were dedicated to the four houses at Hogwarts' wizarding school. More illustrated editions will feature in Bloomsbury's assault on the Christmas market, including two books to accompany a Harry Potter exhibition at the British Library in London. Bloomsbury's glittering performance reflects strong improvements in the British book market.