Which Harry Potter book was the fastest-selling book in history at the time of its 2007 release?

Harry Potter and the Deathly Hallows was the fastest-selling book in history at the time of its 2007 release.

The seventh and final Harry Potter novel was published on 21 July 2007. It sold more than 8 million copies in the United States during its first 24 hours, according to Scholastic, and more than 11 million copies in the United Kingdom and United States combined during the first day.

The book concluded Harry’s conflict with Lord Voldemort and answered major questions established across the series. Its release was accompanied by extensive midnight events, advance reservations, and security measures designed to prevent unauthorized copies from appearing before publication.

Earlier Harry Potter releases had also broken sales records, especially Harry Potter and the Half-Blood Prince in 2005. Deathly Hallows surpassed those figures, making it the record-setting release referred to in this question.