Sarah Ruhl is a mother of three and one of America’s best-known playwrights. She has written a stunningly original book of essays whose concerns range from the most minimal and personal subjects to the most encompassing matters of art and culture. The titles themselves speak to the volume’s uniqueness: “On lice,” “On sleeping in the theater,” “On motherhood and stools (the furniture kind),” “Greek masks and Bell’s palsy.”