The hacker, known only as 'Gabriel', posted messages on the Full Disclosure mailing list at InSecure.org, claiming to be in possession of a full copy of Harry Potter and the Deathly Hallows.
According to the post, 'Gabriel' exploited the PCs of one or more employees at Bloomsbury by sending an email-borne threat.
Once the recipient activated the threat by opening the email, the hacker professes to have been able to lift a draft copy of the highly anticipated book.
"It is amazing to see how much people inside the company have copies and drafts of this book," the so-called hacker said on the mailing list. "Curiosity killed the cat."
'Gabriel' then posted what he claims to be a number of spoilers, revealing the end of the book. "So we make this spoiler to make reading of the upcoming book useless and boring," he said.
Simon Clausen, chief executive at security firm PC Tools, said of the hackers claim: "Is it true? Only the hacker and the publisher really know the answer to that question. Is it possible? Certainly."
Clausen added that companies are being targeted in this way everyday. " Corporate and personal espionage have been taken to a whole new level due to the easy accessibility of the building blocks to create threats," he explained.
"Previously only an experienced hacker would be able to produce custom threats, but these days any tech savvy individual or even 'script-kiddie' can plug the pieces together like Lego and easily make their own."
J K Rowling's final instalment of the Harry Potter saga is not set to hit shelves until 21 July.
"Hackers are regularly using popular events or a newsworthy item to entice users with what is usually a piece of fiction," said Clausen.
"This is another timely reminder to consumers and businesses of the dangers online and why they need to implement protection."
Cotton seedling freezes to death as Chang'e-4 shuts down for the Moon's 14-day lunar night
Fortnite easily out-earns PUBG, Assassin's Creed Odyssey and Red Dead Redemption 2 in 2018
Meteor showers as a service will be visible for about 100 kilometres in all directions
Saturn's rings only formed in the past 100 million years, suggests analysis of Cassini space probe data
New findings contradict conventional belief that Saturn's rings were formed along with the planet about 4.5 billion years ago