* Mystery over who blew Rowling's anonymous book uncovered
* Law firm apologises "unreservedly" for its role
* Author angry over her trust being misplaced
By Belinda Goldsmith
LONDON, July 18 (Reuters) - A real-life mystery over who blew the cover on JK Rowling writing a detective novel under a pseudonym was uncovered on Thursday when the culprit was revealed to be - her law firm, which apologised unreservedly for the leak.
Rowling, whose Harry Potter series made her Britain's best-selling author, posed as a retired military policeman called Robert Galbraith to write "The Cuckoo's Calling" that was released in April to strong reviews but minimal sales.
But the real identity of Galbraith was revealed at the weekend after the Sunday Times newspaper received a set of anonymous tweets alleging Rowling was the author and its staff turned detective to match up the use of language and realise that the same literary agent was involved.
Caught fair and square, Rowling admitted she was Galbraith, sending sales of the detective novel soaring, but one question remained - who sent those tweets in the first place?
Further digging found that the tweets had come from a Twitter user named Jude Callegari but the account was deleted.
Investigations continued and the anonymous tweeter was linked back to Rowling's London-based law firm, Russells, and revealed on Thursday to be the best friend of the wife of one of the firm's partners.
Rowling, in a statement issued by her publicist on Thursday, said she had now discovered how the leak about Galbraith's true identity occurred.
"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," she said.
"To say that I am disappointed is an understatement. I had assumed that I could expect total confidentiality from Russells, a reputable professional firm, and I feel very angry that my trust turned out to be misplaced."
No one from Rowling's side was making any further comment.
Rowling, 47, said it had been "wonderful" to publish for once without hype or expectation and to get feedback under a different name even if that meant some publishers rejected her work as they had when she first touted her Harry Potter books.
Russells apologised "unreservedly" for the disclosure by partner Chris Gossage to his wife's best friend Judith Callegari during a private conversation.
"Whilst accepting his own culpability, the disclosure was made in confidence to someone he trusted implicitly. On becoming aware of the circumstances, we immediately notified JK Rowling's agent," the law firm said in a statement.
"We can confirm that this leak was not part of any marketing plan and that neither JK Rowling, her agent nor publishers were in any way involved."
A spokesman confirmed that Gossage was still a partner but declined to comment on Rowling's relationship with the firm.
"The Cuckoo's Calling" remained at the top of Amazon.co.uk's bestselling list on Thursday for the fourth consecutive day.
The novel had only sold 1,500 hardback copies since being published in April but on Monday raced to the top of bestseller list, leaving high street and online book merchants unable to meet demand.
Publisher Little, Brown, which last year published Rowling's first adult novel "The Casual Vacancy", said it was immediately reprinting "The Cuckoo's Calling" - about war veteran turned private eye Cormoran Strike investigating the death of a model. (Reporting by Belinda Goldsmith; Editing by Michael Roddy)
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