Bloomsbury Publishing said that from 3.45pm until close of business on Thursday, Harry Potter and Prisoner of Azkaban, sold 16,853 copies - overtaking in less than three hours the entire first-day sales of its nearest rival, Hannibal, by Thomas Harris, which sold 12,380.
Prisoner of Azkaban is the third Harry Potter novel by J K Rowling, and the publishers collected extra publicity for it by placing it on sale only after school had closed - reportedly to deter prospective purchasers from playing truant.
"These are just magical books," said a spokeswoman. "Children read them five or six times each and adults laugh out loud when they read them."
The publishers are also celebrating the one millionth sale from the Harry Potter trilogy. The milestone was reached yesterday when an eighth reprint of the latest novel went out, bringing the current run to about 300,000 hardback copies.