Of Lucas's novels the best received was "Cécile" (1930), a tale of love, society and politics in the France of 1775–1776. Lucas dedicated the book to T. E. Lawrence, a friend and admirer. He wrote two further historical novels, "Doctor Dido" (1938), set in Cambridge in 1792–1812, and "The English Agent: A Tale of the Peninsular War" (1969), set in Spain in 1808; and a novella, "The Woman Clothed with the Sun" (1937), on the Buchanites of the 1780s–90s. The three novels focus on a love-affair between an Englishman and a Frenchwoman (Lucas was a self-confessed "gallomane"); the Scots novella takes the form of an account, written by a Scottish minister in middle age, of his youthful bewitchment by Elspeth Buchan and of his curious sojourn among the Buchanites. A theme common to all four is the tension between fragile 18th-century rationalism and, in varying forms, Romantic "enthusiasm" and unreason. For his semi-autobiographical first novel, "The River Flows" (1926), see Personal Life above.