Joan Rivers, at 77, remains one of the hardest-working comedians in the business. She performs more than 200 stand-up gigs each year. Last year, she starred in the second season of NBC’s The Celebrity Apprentice – and to no one’s surprise was the winner. She sells jewelry on cable TV. And she is front and center in Ricki Stern and Anne Sundberg’s fascinating new documentary Joan Rivers: A Piece of Work.