With the seventh and final Harry Potter book to complete, Rowling, at 39, is contemplating her future as a novelist.

Last week she teased fans by disclosing that, although she had several unpublished works “languishing in drawers”, she might write something “completely different”.

She has a strong interest in crime fiction and would be a successful writer of detective novels, according to her friend Ian Rankin, the writer.

The creator of the Inspector Rebus books said: “I know she is a crime fiction fan, so I would love it if she went on and wrote a crime novel. I think the Harry Potter novels are whodunnits anyway. There are lots of red herrings and mystery. As long as she doesn’t set it in Edinburgh I’ll be quite happy.”

Rowling, who is joint 96th in The Sunday Times Rich List, has voiced fears at being typecast as a children’s author. More than 270m Potter books have been sold in 62 languages.

“AA Milne tried to write adult novels and was never reviewed without the mention of Tigger, Pooh and Piglet,” she said recently. “I would imagine the same will happen to me. That’s fine, my shoulders are broad enough.”

Rowling has written two unpublished adult novels and is keen to return to a “liberating” mystery project she worked on between the fourth and fifth Potter books.

“I was writing something else for a while, which was great. I might go back to that,” she said.

Danuta Keane, a former editor of the Bookseller, the publishing trade magazine, said she feared Rowling might struggle to master the demands of more complex, adult emotions in her work.

“I think her first novels were the most successful, before she had to start dealing with teenage emotions. They were much tighter. The last one felt a bit flat. But she is still a great storyteller and adult fiction badly needs good storytellers.”

Keane added that Rowling might struggle to carry many of her existing readers with her. “It will be interesting to see if she attracts a big following after Harry Potter. It can’t be taken as a given.”

Last night a spokeswoman for Rowling refused to be drawn on which genre she intended to pursue. “It is far too early for her to comment in any detail about what she will write next,” she said.

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