After the war Trevelyan worked briefly in merchant banking, then became an editor at William Collins, Sons and later at Jonathan Cape and Michael Joseph, editing both fiction and nonfiction while writing his own books. Among Trevelyan's early popular books were several about Italy, including "Princes Under the Volcano" (1973), an account of the British role in Sicily in the 19th and 20th centuries; and "The Shadow of Vesuvius" (1976), about the discovery of the remains of Pompeii and Herculaneum in the 18th century. A 1978 book, "A Pre-Raphaelite Circle", viewed the major painters of the early Victorian avant-garde movement of the title through the prism of correspondence he found in the papers of Pauline Trevelyan, a distant relation by marriage who was a confidante and patron of the art critic and artist John Ruskin.