A new study confirms what many already knew: Oprah Winfrey's (search) book endorsements are good as gold to publishers.

"Oprah's recommendations had a bigger impact on the sales of books than anything we have previously seen in literature, or seen since," said Brigham Young University economics professor Richard Butler (search), whose findings were published in the latest issue of the journal Publishing Research Quarterly (search).

Butler found that Winfrey's recommendation was enough to lift books from obscurity and to keep them on the best-seller lists longer than other titles.

Using USA Today's weekly 150-item best-seller list, Butler and his team of students went about examining the 45 non-children's titles Winfrey picked from her book club's inception in 1996 until she announced its end in 2002.

Of those books, only 11 had been on the best-seller list before her recommendation, and none of them had gone beyond No. 25. Of the first 11 books that Winfrey picked, all went to at least No. 4 within a week, Butler said.

Among those most affected were "Poisonwood Bible" by Barbara Kingsolver, which was on the best-seller list for 137 weeks, and Billie Letts' "Where the Heart Is," which lasted 98 weeks.

Winfrey restarted the picks last summer but now recommends only classics.