SHEILAH BECKETT (born 1913) turned 100 shortly after finishing her illustrations for "The Nutcracker." She has brought to life dozens of children's books in her long career, including the recently reissued Little Golden Book "The Twelve Dancing Princesses, "originally published in 1954. Entirely self-taught, she was the first woman to work at the prestigious Charles E. Cooper advertising art studio, where she illustrated advertisements, album covers, and Christmas cards. But illustrating fairy tales has always been her first love, and is what she's best known for.