As many as 1,200 copies were shipped early in the US by an online retailer, and the New York Times and the Baltimore Sun published reviews of the book earlier this week.
Rowlin, however, said she was "staggered" by the embargo-busting reviews.
"I am incredibly grateful to all those newspapers, booksellers and others who have chosen not to attempt to spoil Harry's last adventure for fans," she said.
New York Times book editor Rick Lyman defended the newspaper's decision to run its review before publication.
"Our feeling is that once a book is offered up for sale at any public, retail outlet, and we purchase a copy legally and openly, we are free to review it," a spokeswoman said.
In the Times review, writer Michiko Kakutani gives away some plot details, including roughly how many characters die and what "deathly hallows" means, but does not leak the big secrets.
Potter publishers will take comfort from the fact that the majority of fans do not know what happens in book seven, and do not want to until they get their hands on a copy on what has been dubbed in the media as "P-Day."
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