How do Shakespeare's Twelfth Night and Sonnet 73 cover the theme don't judge a book by its cover?It's a short answer question.
Sonnet 73 is about someone who loves a person who is getting old. According to the speaker in the sonnet, the love is stronger because the lover recognizes that they don't have long together. It is about someone who loves a person despite his outward appearance. So, people who are not judging the book by the cover.
In 12th Night, what is below the surface is more important than what is above. Olivia and Orsinio both judge books by their covers, so to speak, when they fall in love with people who are attractive on the outside but wrong for them in one way or another. Orsino likes Olivia's looks but he doesn't see (or care to see her character) and Olivia falls in love with Viola without realizing the "boy" she's fallen for is actually a girl. The truest love is the one between Orsinio and Viola for that is the love of friends who recongize and understand each other's true character. That kind of love is one that judges from the inside rather than outside appearances.