Initially, Clark collected a broad array of English imprints. His library included the four Shakespeare folios; important editions of Chaucer, Ben Jonson, Byron, Dickens, and Robert Louis Stevenson; works illustrated by George Cruikshank and William Blake; French literature from Pierre de Ronsard to Émile Zola; autograph letters and manuscripts by authors, statesmen, and musicians; and materials relating to the exploration of the American West. In time, Clark began to concentrate his collecting on English literature of the 17th and 18th centuries, particularly in the Restoration, which defines the strengths of the Clark Library today. Eventually, Clark also developed a large collection of Oscar Wilde books and manuscripts.