"It's a blow," said Lorraine Shanley, a partner in the consulting firm Market Partners International Inc., who earlier this week watched former Alaska governor and Republican vice presidential candidate Sarah Palin promote her book on Ms. Winfrey’s show.

"Oprah Winfrey has supported many authors, and her book club has had a huge impact on America's reading habits," added Ms. Shanley. "She made Faulkner a best seller again. She also promoted an eclectic group of authors and created publishing successes for many commercial writers."

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Over the years, the book club has helped to bring the works of well-known writers such as Maya Angelou, Toni Morrison and Cormac McCarthy to even broader audiences. The show has also introduced writers such as Edwidge Danticat to millions of readers.

"Other than a book being turned into a popular movie nothing brings readers to a book like Oprah," said Dawn Davis, editorial director of the Amistad imprint of News Corp.'s HarperCollins Publishers. (News Corp. also owns The Wall Street Journal.) "She brings a variety of readers to a variety of books. Her impact is immeasurable."