Harry Potter is continuing to work his magic for publisher Bloomsbury 20 years after his debut, with special editions of his first adventure helping revenue rise 15%.

Bloomsbury released new editions of J.K. Rowling's "Harry Potter and the Philosopher's Stone" with covers dedicated to wizarding school Hogwarts' houses Gryffindor, Slytherin, Hufflepuff and Ravenclaw to mark the anniversary in June.

The new designs helped Bloombury's children's trade division deliver what it said was "another outstanding" performance, increasing revenue by a third in its first half.

Revenue from Harry Potter grew 40% in the period, the company said today, and the young wizard and his magical world will feature strongly in the publisher's assault on the important Christmas market.

"We have a strong second-half list including the illustrated edition of 'Harry Potter and the Prisoner of Azkaban,' the illustrated edition of 'Fantastic Beasts and Where to Find Them' and two major books to accompany the British Library's Harry Potter exhibition," said chief executive Nigel Newton.

The robust performance from Bloomsbury reflects a wider improvement in the British book market, where special editions featuring vintage covers and new artwork have helped draw readers back to print formats.