This 1923 edition made his reputation as a book illustrator, during the golden age of gift-book illustration in the first quarter of the twentieth century. Clarke's work can be compared to that of Aubrey Beardsley, Kay Nielsen, and Edmund Dulac. It was followed by editions of "The Years at the Spring", with 12 colour plates and more than 14 monotone images; (Lettice D'O. Walters, ed., 1920), Charles Perrault's "Fairy Tales of Perrault", and Goethe's Faust, with eight colour plates and more than 70 halftone and duotone images (New York: Hartsdale House,1925). The last of these is his most famous work, prefiguring the imagery of 1960s psychedelia. Two of his most sought-after titles are promotional booklets for Jameson Irish Whiskey: "A History of a Great House" (1924, and subsequent reprints) and "Elixir of Life" (1925), which was written by Geofrey Warren. His final book, "Selected Poems of Algernon Charles Swinburne", was published in 1928. Clarke produced more than 130 windows, he and his brother Walter having taken over his father's studio after his death in 1921. His glass is distinguished by the finesse of its drawing and his use of rich colours, and an innovative integration of the window leading as part of the overall design, originally inspired by an early visit to see the stained glass of the Cathedral of Chartres. He was especially fond of deep blues. Clarke's use of heavy lines in his black-and-white book illustrations echoes his glass techniques. Clarke's stained glass work includes many religious windows, but also much secular stained glass. Highlights of the former include the windows of the Honan Chapel in University College Cork; of the latter, a window illustrating John Keats' "The Eve of St. Agnes" (now in the Hugh Lane Municipal Gallery in Dublin) and the "Geneva Window", created for the Centre William Rappard in Geneva, Switzerland (now in the Wolfsonian Museum, Miami, Florida, USA). Perhaps his most seen works were the windows he made for Bewley's Café on Dublin's Grafton Street. Both Harry and his brother Walter were plagued with ill health, in particular problems with their lungs. Clarke was diagnosed with tuberculosis in 1929, and went to a sanatorium in Davos, Switzerland.