“Michael V. Korda will step down at the end of the year as editor in chief of Simon & Schuster’s trade books imprint, a post he has held since 1968, the company said yesterday. He will remain as editor in chief emeritus, editing the books of about a half dozen writers, including David McCullough, Larry McMurtry and Mary Higgins Clark.” The move appears to be entirely voluntary on Mr. Korda’s part.