As his personal fortune grew, Clements cultivated a passion for history and rare books, concentrating on Americana, particularly books written contemporaneously with the events they describe—the primary sources for early American history. Clements entered the rare book world at the end of a “golden age” for collectors. The decades from the 1880s through the 1920s saw the breakup of the British estate system, and the arrival at auction or sale of the contents of many aristocrats’ great libraries. At the same time in the United States, a series of great private libraries were sold. With these fortuitous conditions in the rare book market, and with the advice of expert dealer/bibliographers, Clements made a series of major purchases, concentrating on two periods of American history—the discovery, exploration, and colonization of America by Europeans from the 15th through the 17th centuries, and late 18th century colonial and revolutionary America.