An exhibit designed as career retrospective must be a daunting proposition, especially when focused on a prolific artist whose output ranged over sixty years. Too many pieces, and the exhibit becomes bloated. Too few, and the audience does not get an accurate cross section of the artist’s work. Thankfully, the curators of Saul Steinberg: Illuminations, on display at the Smithsonian American Art Museum, threaded this needle. In doing so, they have mounted a captivating and…