Agents for author JK Rowling have sent legal warnings to websites hosting unauthorised electronic copies of Harry Potter and the Half-Blood Prince.
Parties were held all over the world to celebrate the book's launch
Electronic versions of the new Harry Potter book began appearing on the internet within 12 hours of the sixth instalment going on sale on Saturday.
Pirated audio versions of the hugely-popular book are also available on file-sharing networks.
Agents Christopher Little said they were working to combat piracy.
"We have put in place some countermeasures but I can't disclose the specifics," said Neil Blair, a spokesman for the literary agency.
"We have sent takedown notices but we haven't filed any lawsuits," he added.
Harry Potter and the Half-Blood Prince went on sale across the world on Saturday, selling more than 8.9 million copies in the first 24 hours.
Author JK Rowling has yet to make a deal to publish her books in a downloadable format, and has hit out at unauthorised Harry Potter books on her website.
"You should never trust any Harry Potter e-books offered for download from the internet or on P2P/file-trading networks," she wrote in January, warning they may also expose users to computer viruses or fraud.