Not much is known about Janet Halverson, who designed the book covers for titans of the 20th century literary scene: She was born in Maine in 1926; spent her youth in New Haven; worked in New York; now, aged 97, lives in New Jersey. A new exhibition at Katherine Small Gallery allows her work to speak for her, highlighting her designs for a number of striking book covers. A coiled silhouette of a snake, almost an @ sign, has its tongue aiming up at a glowing sundown against a fuchsia band for Joan Didion’s “Play It As It Lays.” The...