Christopher Little said Rowling had "retained all the rights in the 'Harry Potter' series," including any companion books "which she may indeed write herself," The Sunday Telegraph reported.

Rowling, 41, is reportedly planning to use her background notes from the past 10 years to write an encyclopedia on the world of Harry Potter.

The book would cover "historical" figures, including the founders of Hogwarts School for Witchcraft and Wizardry, and present elaborate genealogies of the main characters.

Rowling has said it is "highly unlikely" that she would write any more "Harry Potter" books, she announced on her Web site there is a chance she might go with the encyclopedia idea.

"I might do an eighth book for charity, a kind of encyclopedia of the world so that I could use all the extra material that's not in the books," she wrote on JKRowling.com.