The novel reveals events from the early life of Lord Voldemort, and chronicles Harry's preparations for the final battle against him.

It sold almost seven million copies in the first 24 hours after its release,[1] a record eventually broken by its sequel, Harry Potter and the Deathly Hallows.

As Albus Dumbledore escorts him to the Weasley home, they visit the retired Hogwarts professor Horace Slughorn, who agrees to resume teaching.

While a student at Hogwarts, Riddle had asked Slughorn about objects called Horcruxes, which can grant immortality by encasing fragments of a wizard's soul.

Slughorn's memory suggests that Voldemort created six Horcruxes, though Dumbledore explains that two are already destroyed: his diary and a ring his grandfather owned.

In a remote cave, Harry and Dumbledore overcome many obstacles before seizing the Horcrux, the locket of Hogwarts founder, Salazar Slytherin.

A number of high-value bets were made on the death of Albus Dumbledore, many coming from the town of Bungay where it was believed the books were being printed at the time.

[27] In early July 2005, a Real Canadian Superstore in Coquitlam, British Columbia, Canada, accidentally sold fourteen copies of The Half-Blood Prince before the authorised release date.

[28] On 15 July, less than twelve hours before the book went on sale in the Eastern time zone, Raincoast warned The Globe and Mail newspaper that publishing a review from a Canada-based writer at midnight, as the paper had promised, would be seen as a violation of the trade secret injunction.

The Christian Science Monitor's reviewer Yvonne Zipp argued the first half contained a lighter tone to soften the unhappy ending.

[33] The Boston Globe reviewer Liz Rosenberg wrote, "lightness [is] slimmer than ever in this darkening series...[there is] a new charge of gloom and darkness.

[35] Liesl Schillinger, a contributor to The New York Times book review, also noted that Half-Blood Prince was "far darker" but "leavened with humor, romance and snappy dialogue."

[36] David Kipen, a critic of the San Francisco Chronicle, considered the "darkness as a sign of our paranoid times" and singled out curfews and searches that were part of the tightened security at Hogwarts as resemblances to our world.

[37] Julia Keller, a critic for the Chicago Tribune, highlighted the humour found in the novel and claimed it to be the success of the Harry Potter saga.

[38] Rosenberg wrote that the two main themes of Half-Blood Prince were love and death and praised Rowling's "affirmation of their central position in human lives."

[34] Zipp noted trust and redemption to be themes promising to continue in the final book, which she thought "would add a greater layer of nuance and complexity to some characters who could sorely use it.

"[36] Kirkus Reviews said it "will leave readers pleased, amused, excited, scared, infuriated, delighted, sad, surprised, thoughtful and likely wondering where Voldemort has got to, since he appears only in flashbacks."

"[43] Yvonne Zipp of The Christian Science Monitor praised the way Rowling evolved Harry into a teenager and how the plot threads found as far back as Chamber of Secrets came into play.

[34] The Associated Press writer Deepti Hajela praised the newfound emotional tones and ageing Harry to the point at which "younger fans may find [the series] has grown up too much.

"[39] Emily Green, a staff writer for the Los Angeles Times, was generally positive about the book but was concerned whether young children could handle the material.

[44] Cultural critic Julia Keller of the Chicago Tribune called it the "most eloquent and substantial addition to the series thus far" and considered the key to the success of the Potter novels to be humour.

[55] The US audiobook, read by Jim Dale, set sales records with 165,000 sold over two days, besting the adaptation of Order of the Phoenix by twenty percent.

[56] Harry Potter and the Half-Blood Prince was published simultaneously in the UK, US, Canada, Australia, New Zealand, and South Africa.

[63] To celebrate the release of the American paperback edition, Scholastic held a six-week sweepstakes event in which participants in an online poll were entered to win prizes.

[66] Also released on 16 July was the Scholastic "Deluxe Edition," which featured reproductions of Mary GrandPré's artwork and had a print run of about 100,000 copies.