Having completed his term he came to London in 1818 and entered the establishment of Ogle, Duncan, & Cochran, 295 High Holborn, who carried on a trade in theological books.
Darling compiled in 1843 the ‘Bibliotheca Clericalis, or the Catalogue of the Books in the Clerical Library and Reading Rooms, 21, 22, and 23 Little Queen Street, Lincoln's Inn Fields,’ a volume of 316 pages, giving an abstract of the contents of all the major works.
In 1851 he brought out the first part of the ‘Cyclopædia Bibliographica, or Library Manual of Theological and General Literature: Authors,’ a rival to the works of Robert Watt and William Thomas Lowndes.
It contains the names of notable theological authors, gives a short biographical or descriptive notice of their writings, and then an analysis of each volume.
Another work bearing his name is ‘Catalogue of Books belonging to Sir William Heathcote at Hursley Park, 1834,’ lithographed in imitation of manuscript.