J. Nicholson 

J. Nicholson was a printer/bookseller in Bradford, Yorkshire.  In 1801 he served as Flower’s agent for the Intelligencer.  He may have been a relation of John Nicholson, Jr., a Cambridge bookseller whose family had been in business for nearly fifty years at the time of the above letter (Cambridge Intelligencer, 13 August 1796). The latter Nicholson sold several books printed by Flower during his years in Cambridge.