BOOKSELLERS rebelled at the original cover for "The Surrender: An Erotic Memoir" by Toni Bentley. ReganBooks boss Judith Regan wanted to use a photo of a woman's posterior barely covered in sheer panties. But Regan was persuaded to bury the butt shot under a stark black dust-jacket. Meanwhile, the words inside come off like an intellectualized letter to Penthouse magazine. The author, a former ballerina, describes her first painful experience of anal sex as "an anatomical miracle ... If I had walked on water I couldn't have been more amazed. This was my first act of sacrifice that was not mired in the vicious circle of self-reflective narcissism ... I have been changed ever since."