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

Simon & Schuster announced yesterday 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.

Publishers and authors have worried that e-books might hurt sales for hardcovers, which cost more; Amazon.com, Barnes & Noble.com and other online retailers commonly price top-selling e-releases at $9.99.

With the digital market estimated at between 2 and 5 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."