NEW YORK (AP) - 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released Tuesday, moved up and down like stock market shares as rivals Amazon.com and Walmart.com extended, then rescinded, their high discounts for top-selling pre-orders.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.

"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," Walmart.com 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 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 our product," Grisham told The Associated Press on Tuesday.

Target.com was selling "Ford County" for $15 on Tuesday. Online sellers also raised prices for two other releases that came out Tuesday and had been offered for $9 or less as pre-orders: J.D. Robb's "Kindred in Death" and Barbara Kingsolver's "The Lacuna."

The price of e-books, which Amazon.com and others often sell for under $10, was another concern for Grisham. Although the author says he owns a Kindle and believes digital reading devices will keep getting "better and cooler and smarter," he believes that best-selling releases cost so little 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 currently does not have a release date for the electronic version. "We may do something like that (for `Ford County')."