Complex sentence: In 1890, he became literary adviser for D. Appleton & Company, in which capacity he edited Edward Noyles Wescott's narrative "David Harum" (1898) into a bestseller, later made into a film.
Split into simpler sentences: In 1890, he became literary adviser for D. Appleton & Company. In that capacity, he edited Edward Noyles Wescott's narrative "David Harum" (1898) into a bestseller, later made into a film.