In 1860, Lecky published anonymously a small book entitled "The Religious Tendencies of the Age", but on leaving college he turned to historiography. In 1861 he published "Leaders of Public Opinion in Ireland", containing brief sketches of Jonathan Swift, Henry Flood, Henry Grattan and Daniel O'Connell, originally anonymous, republished in 1871; the essay on Swift, rewritten and amplified, appeared again in 1897 as an introduction to an edition of Swift's works. Two surveys followed: "A History of the Rise and Influence of Rationalism in Europe" (2 vols., 1865), and "A History of European Morals from Augustus to Charlemagne" (2 vols., 1869). The latter aroused criticism, with its opening dissertation on "the natural history of morals." Lecky's "History of European Morals" was one of Mark Twain's favorite books, and influenced the writing of "A Connecticut Yankee in King Arthur's Court".