JK Rowling is “very angry” at the law firm that she trusted with her pen name. After she was revealed to be the author of a new crime novel, The Cuckoo’s Calling, using the distinctly male pseudonym of Robert Galbraith, the Harry Potter writer found out where the leak came from.

Earlier in July, The Sunday Times have revealed that the highly acclaimed but average-selling new novel that was published in April was written by no other than Ms Rowling herself.

“I had hoped to keep this secret a little longer, because being Robert Galbraith has been such a liberating experience. It has been wonderful to publish without hype or expectation, and pure pleasure to get feedback under a different name,” she had told the UK paper that time.

The distinguished author then found out that the British law firm Russells was responsible for exposing her identity.

“I feel very angry that my trust turned out to be misplaced,” she told The Sunday Times, via the BBC. “To say that I am disappointed is an understatement.”

“A tiny number of people knew my pseudonym and it has not been pleasant to wonder for days how a woman whom I had never heard of prior to Sunday night could have found out something that many of my oldest friends did not know.”

The Cuckoo’s Calling was published under Sphere, the same imprint at Little, Brown Group that also published her non-Harry Potter book, The Casual Vacancy.

For its part, Russells Solicitors has offered their apology “unreservedly.”

The law firm said in a statement that the wife of one of its partners, Chris Gossage, had told her best friend, Judith Callegari, of the book’s real authorship.

It added that it was leaked “during a private conversation” and that it was “made in confidence to someone he trusted implicitly.”

The law firm continued, “On becoming aware of the circumstances, we immediately notified JK Rowling’s agent.”

It is understood that Ms Callegari revealed Galbraith’s real identity to a Sunday Times journalit via Twitter on July 9.

Since Ms Rowling’s outing, the detective book’s sales have gone up to 156,866 per cent on Amazon. According to the New Statesman, it climbed up from 4,709 rank on the Web site’s best sellers list to number three.

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JK Rowling Admits To Using Pen Name Robert Galbraith To Sell New Book, Sales For ‘The Cuckoo’s Calling’ Shot Up After Revelation [Read]