He then began to work as an editor for the publishing house Simon & Schuster, where after eight years he rose to senior editor, becoming known as "the house brain", handling brainier authors including British philosopher Bertrand Russell, "Zorba the Greek" author Nikos Kazantzakis, and sociologist C. Wright Mills. Fascinated by words and language, by his early 20s Kaplan had edited translations of Plato and Aristotle. In his memoir Back Then (2002) Kaplan wrote: "It was fun to work at Simon & Schuster. [It was] not surprising to see editors staying long after hours to talk books, trade industry gossip, and joke over office bottles of Scotch and gin. In the days before it was absorbed into a conglomerate the house was like a summer camp for intellectually hyperactive children", only without a curfew, reminiscing about dancing at a party with Marilyn Monroe, "gently kneading the little tire of baby fat around her waist."