Thomas Ryder plans to retire as chief executive of Reader's Digest ( RDA Jan. 1, but he's been asked by the board to remain chairman through next year. Eric Schrier, global editor-in-chief and president of the company's North American operation, will become Reader's Digest's president and CEO. Additionally, Thomas Gardner, president of Reader's Digest International, will be named to the new position of executive vice president. The Pleasantville, N.Y., company said Monday that Schrier, 53, has been responsible for all North American business operations, including Reader's Digest magazine, Reiman Publications, U.S. Books and Home Entertainment and Reader's Digest Canada. He joined the company, whose wholesome namesake magazine can be found in supermarket checkout aisles everywhere, in January 2000. Ryder, 61, left American Express ( AXP in 1998 to join Reader's Digest.