The cover of "The Cuckoo's Calling," a crime novel written by JK Rowling and released under a pseudonym. Mulholland Books

Back in April, Mulholland Books released the obscure detective novel “The Cuckoo’s Calling” to good reviews, but less-than-noteworthy sales. On Sunday, however, sales of the book jumped more than 500,000 percent on Amazon, sending the book to the top of the best sellers list. What changed? “The Cuckoo’s Calling” author Robert Galbraith was revealed to be nothing more than a pseudonym for “Harry Potter” author J.K. Rowling.

When “The Cuckoo’s Calling” was released in April, it was billed as a hard-boiled detective novel from first-time author Robert Galbraith. Mulholland described the so-called Galbraith as a married, ex-military father who now works as a civilian security consultant. “The Cuckoo’s Calling” was reported as his first novel. The publisher acknowledged that Robert Galbraith was a pseudonym, but most reporters had little reason to suspect that Rowling was behind the book.

"He left the military in 2003 and has been working since then in the civilian security industry," the publisher's website said. "The idea for (protagonist) Cormoran Strike grew directly out of his own experiences and those of his military friends who returned to the civilian world. 'Robert Galbraith' is a pseudonym."

The book received universal acclaim when it was released, yet it seemed to fly largely under the radar. Despite the book's relative obscurity, however, it made a big enough impression that questions about the authorship of "The Cuckoo's Calling" remained in critics' minds for months.

Rumors that Rowling had actually written “The Cuckoo’s Calling” began to spread on Thursday, after a friend of New York Times arts editor Richard Brooks received a tip about the book’s true author. Brooks’ friend reportedly tweeted the “The Cuckoo’s Calling” did not seem like the work of a first-time novelist, and soon after received an anonymous tweet that the book was actually a side project for Rowling.

Soon after, Brooks noticed that Rowling and "Galbraith" shared the same publisher, the same agent and the same editor. Suspicious about the author, he sent copies of “The Cuckoo’s Calling,” “Harry Potter and the Deathly Hallows,” and Rowling’s latest adult novel, “The Casual Vacancy” to a “pair of computer linguistic experts” who confirmed notable similarities between the books’ writing styles. After presenting the evidence to Rowling’s publicist, the “Harry Potter” author owned up to the truth.

On Sunday, Rowling confirmed her authorship of “The Cuckoo’s Call” in a public statement.

"I hoped to keep this secret a little longer, because being Robert Galbraith has been such a liberating experience!" Rowling said in a statement. "It has been wonderful to publish without hype or expectation and pure pleasure to get feedback from publishers and readers under a different name.”

“The Cuckoo’s Calling” is currently a No. 1 bestseller in both the United States and Britain. Sales of the book — which had originally only sold 1,500 copies — have shot up 507,000 percent.