(AP Photo/Bebeto Matthews) ?We run the risk of seriously devaluing out product,? author John Grisham said Tuesday, adding that he was unhappy about the price of e-books, which Amazon.com and others often sell for under $10. (The Associated Press)

New York » It was a weird day in the book price wars.

The cost of John Grisham's Ford County , officially released Tuesday, moved up and down like stock market shares as rivals Amazon.com and Walmart.com extended, then rescinded, their high discounts for top-selling pre-orders.

Early in the day, Amazon was selling Grisham's book of short stories for $9, the same price it had offered for Ford County before publication and a sign that Amazon was ready to continue the cost competition beyond the release date. Walmart.com was selling Ford County for $12 early Tuesday, then cut the price to the pre-order discount of $8.98.

By Tuesday afternoon, the cost was back up to $11.99 for both online sellers. Target.com was selling Ford County for $15 on Tuesday.

"We remain committed to offering our customers the lowest prices online at Walmart.com, and will continue to adjust our pricing accordingly and as needed on these books," said spokesman Ravi Jariwala.

Amazon.com did not immediately respond to requests for comment.

Hardcovers generally have a list price of $24 or higher, but in mid-October Walmart.com announced dramatic cuts for pre-orders of Ford County , Sarah Palin's Going Rogue and other popular November publications.

Wal-Mart.com initially charged $9 for the books, a price quickly matched by Amazon.com, then it dropped to just

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under $9 by Wal-Mart.com and Target.com. Authors, publishers and rival booksellers worry that cutting the price so low will harm competition and force down the cost of books overall, leading to a reduction in author advances.

"We run the risk of seriously devaluing out product," Grisham said Tuesday, adding that he was unhappy about the price of e-books, which Amazon.com and others often sell for under $10. Although the author says he owns a Kindle and believes digital reading devices will keep getting "better and cooler and smarter," he believes that the price of e-books for new releases is so low that hardcover sales might suffer.

Ford County joins a growing list, including Palin's memoir and Stephen King's Under the Dome , of books for which the e-edition will not come out until well after the hardcover.

"When a movie comes out you go see it and three later months later you buy the DVD," said Grisham, whose book does not have a release date for the electronic version. "We may do something like that" for Ford County .