Harper Lee's Go Set a Watchman has become the most pre-ordered title on Amazon since the final Harry Potter book, the online retailer has said.
The opening chapter of the highly-anticipated novel by the reclusive author of To Kill A Mockingbird was published for the first time on Friday.
The novel, which is currently the number one best-seller on Amazon.com, is due to be published on 14 July.
Amazon did not give any figures for pre-orders, but publisher HarperCollins said it had ordered an initial US print run of two million.
Lee wrote Go Set a Watchman in the mid-1950s - but her editor persuaded her to turn some of the story's flashback sequences into a separate novel.
That novel became To Kill A Mockingbird, which went on to win the Pulitzer Prize in 1961 and has sold more than 40 million copies worldwide.
Go Set a Watchman was discarded and was only rediscovered when Lee's lawyer found it it with the original To Kill a Mockingbird manuscript in a safety deposit box.
The first chapter opens with Jean Louise Finch - nicknamed Scout in To Kill A Mockingbird - in her 20s and on a train from New York to visit her ailing father Atticus in their home town of Maycomb, Alabama.
It sees her weighing up a marriage proposal from her brother's friend Henry Clinton and preparing to face her father's illness.
Jean Louise Finch is described as having "turned from an overalled, fractious, gun-slinging creature into a reasonable facsimile of a human being".
'Restlessness of spirit'
Lee wrote that "although she still moved like a 13-year-old boy and abjured most feminine adornment, he [Henry] found something so intensely feminine about her that he fell in love".
"She was easy to look at and easy to be with most of the time, but she was in no sense of the word an easy person.
"She was afflicted with a restlessness of spirit he could not guess at, but he knew she was the one for him."
HarperCollins said the novel goes on to see Jean Louise struggle "with issues both personal and political, involving Atticus, society, and the small Alabama town that shaped her".
Jonathan Burnham, publisher of HarperCollins's Harper imprint, said the text had been altered little from the 1950s manuscript.
"It was made clear to us that Harper Lee wanted it published as it was," he said. "We gave the book a very light copy edit."
Lee has largely avoided the spotlight since the 1960s and is not expected to appear in public to promote the novel. She has poor hearing and vision and lives in an assisted living facility in her home town of Monroeville, Alabama.