You can’t tell a book by its cover, or a mattress for that matter. Once Upon a Mattress is NOT the title of a racy play featuring a porno star. It’s a delightful Off-Broadway musical that brought stardom to the young Carol Burnett as Winifred the Woebegone. The unknown Eliot Gould also was in the original cast. Once Upon a Mattress is a clever retelling of The Princess and the Pea, smartened up for adults although kids will enjoy it, too. It’s part of Northwestern University’s summer theater festival, through August 14. Barber Theatre, 30 Arts Circle Drive, Evanston campus; (847) 491-7282; $27.