A spokesman for Christie's auction house says it's a new world record for a historical document.
Christie's says the bidder was a representative of the nonprofit educational organisation that operates the museum at Washington's Virginia home.
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The 223-year-old book has President of the United States on the cover and also contains a copy of the Bill of Rights. Washington added brackets and notes that highlighted key passages about the responsibility of the president.
It sold for $US9.8 million ($A9.78 million) on Friday to the Mount Vernon Ladies' Association of the Union. The book will be returned to the library at Mount Vernon.
