Critics have been split on the latest Harry Potter, with comments ranging from "pedestrian" to a "classic" in the mould of Lord of the Rings.
The book went on sale at 0001 on 16 July
The 607-page Harry Potter and the Half-Blood Prince was published around the world at 0001 on 16 July.
Suzy Feay in the Independent on Sunday called JK Rowling's effort "wordy, flabby and not very well edited".
The New York Times' Michiko Kakutani equated the book's achievement to classics such as the Lord of the Rings.
She called it "the creation of a richly imagined and utterly singular world, as detailed, as improbable and as mortal as our own".
Robert McCrumb writing in The Observer said her "prose ran the gamut from torpid to pedestrian".
But he added "her plot - driven by the quest for the identity of the Half-Blood Prince - always clips along inventively".
Many newspapers invited younger readers to the review the book, with a mixed response from its target audience.
Author JK Rowling was at Edinburgh Castle for the launch
In the Mail on Sunday, James Noble, 10, wrote: "I thought the beginning of the book was a bit boring because all the characters were doing was talking."
'Chaotic and action-packed'
Rosie Jenkins, 10, was one of the lucky winners who got to meet Rowling at the book's launch at Edinburgh Castle.
She said the book "immediately plunges the reader into a world that is grim, chaotic and action-packed".
She added it was "darker and more alarming than the others but that makes it more interesting and impossible to put down".
BBC News website's Darren Waters read the Half-Blood Prince in less than six hours.
He said: "Too much of the book was either a repeat of what we have seen before, or bogged down by Rowling's attempts to manoeuvre plot lines and characters into position.
"After a while all magic tricks begin to lose their impact."
Negative reviews are unlikely to dent the massive sales for the book, with 10 million copies predicted to have sold around the world in its first 24-hours.
The first five Harry Potter books sold more than 265 million copies in 200 countries and have been translated into 62 languages.
On Sunday, ITV is to screen an exclusive interview with Rowling, conducted by 14-year-old Owen Jones of Cardiff.
He won the prize to be the only British person to interview the author on her latest book.