NEW YORK (AP) — As the market quickens for "e-books," the schedule for their release is slowing down.

Simon & Schuster announced on Wednesday that the electronic editions for more than 30 works coming out in the first half of 2010 would not be available until four months after the hardcover. The affected books include novels by Don DeLillo and Mary Higgins Clark and a memoir by Karl Rove.

HarperCollins spokeswoman Erin Crum said the publisher plans to withhold the digital version of five to 10 titles per month, starting next year. And Hachette Book Group USA will delay a "wide selection" of e-books in 2010, spokeswoman Sophie Cottrell said.

Publishers and authors have worried that e-books might hurt sales for hardcovers; Amazon.com, Barnes & Noble.com and other online retailers commonly price top-selling e-releases at $9.99, which publishers say is too low and could cheapen the value of books overall.

Simon & Schuster CEO Carolyn Reidy said Wednesday that the rise of e-books has led to a "cannibalising" of new hardcover purchases.

"We believe that a large portion of the people who have bought e-readers are from the most devoted reading population," Reidy said. "And if they like the e-readers they are naturally going to convert because the e-books are so significantly less expensive."

E-books do expand the market for older books, Reidy added. Readers buying a new fiction work are inspired to buy many of the same author's previous books, or "backlist," in digital format because of the ease of ordering. Previously, buyers might have gone to a retail outlet and bought just one backlist title, she said.

With the digital market estimated at two to five percent of total sales, more than double from two years ago, e-books were held back for several of the fall's leading titles, including the late Sen. Edward Kennedy's "True Compass," Sarah Palin's "Going Rogue" and Andre Agassi's "Open."

The e-book for the season's most popular release, Dan Brown's "The Lost Symbol," came out at the same time as the hardcover and has sold about 200,000 copies, or about five percent of the book's total sales, nearly unthinkable before the rise of the Kindle and other digital devices.

"Authors get the most publicity at launch and need to strike while the iron is hot," Amazon spokesman Andrew Herdener said Wednesday. "If readers can't get their preferred format at that moment, they may buy a different book or just not buy a book at all."