By DAVID SHEETS Poet and bestselling author Mary Karr, who will speak Friday at UNT's Mayborn Literary Nonfiction Conference, says the personal things readers tell her at book signings are something "sacred.""Because so many people know so much about what happened in my life, they sometimes feel comfortable telling me things they wouldn't tell most people," Karr says.The author of three memoirs, she says she enjoys hearing about others' lives and struggles.