Mary Ruefle’s essay “On Imagination’’ is the latest offering in Sarabande’s Quarternote Chapbook series, which has featured work by Louise Glück, Lydia Davis, Frank Bidart, and James Tate. Ruefle’s dissection of imagination showcases her good, wild mind — it feels as though sitting fireside with a friendly sage willing to offer up the contents of her brain. She writes of her relationship with imagination, its one-ness with thought, its bright and dark sides. She brings Gertrude Stein and Wittgenstein into the conversation, as well as Robert Frost and Johnny Cash. This piece, like much of her work, is erudite, chatty, and deeply engaged with the world and the crackling life of the mind.