The Sunday Times broke the story first: JK Rowling released a book in April under the pseudonym of Robert Galbraith. It is called Cuckoo's Calling and it has received good reviews. A Publisher's Weekly reviewer starts their review with "In a rare feat, the pseudonymous Galbraith combines a complex and compelling sleuth and an equally well-formed and unlikely assistant with a baffling crime in his stellar debut."

Update: Amazon (U.S. and U.K.) have just sold out of their hardcover copies... now listing a 10-14 day wait.