This collection of bits of wisdom begins with Oprah Winfrey recalling a 1998 interview during which the late, great film critic Gene Siskel asked her, “[W]hat do you know for sure?” ...

having coauthored five, including one about weight loss that she penned with her personal trainer Bob Greene. Meanwhile, What I Know for Sure is scheduled to hit shelves on Sept. 2.

Oprah was once asked on live TV what she knew for sure. Now, 16 years later she has an answer and she's revealing it to ET. PICS: Must-See Celeb Sightings "What I know for sure is no matter what ...