Harris earned his income as an artist chiefly from the production of facsimiles, used to replace pages (or entire sections) of books which had been damaged or become decayed. Harris himself suggested that the process of replacing damaged pages with facsimile reproductions was first popularised by
George Spencer, 2nd Earl Spencer, who commissioned such work through Harris's employer, John Whittaker. During his time working at the British Museum (from 1820 onwards), Harris repaired or replaced sections of many books in the national collection, and contemporary reports state that his facsimile work was entirely indistinguishable from the original. A British Museum colleague, Robert Cowtan, gave an account of the examination of a book from the national collection by the British Museum's foremost book experts, Messrs Panizzi, Jones, and Watts, and their abject failure to discover the reproduced sections. "After a fruitless search, page by page, the consultation ended in a summons to Mr. Harris himself to point out the leaves that he had supplied." Cowtan also stated of Harris, "In this curious art he is probably unrivalled....and some of the leaves that he has supplied are so perfectly done that, after a few years, he has himself been puzzled to distinguish his own work from the original, so perfect has the facsimile been, both in paper and typography." It was because of the success of Harris's work that the Trustees of the British Museum enacted a regulation that all facsimile pages added to restored books must be marked with a symbol at the base of the page, so that the librarians of future generations would be able to distinguish original pages from reproductions. It is known that Harris carried out book restoration work for
Prince Augustus Frederick, Duke of Sussex, the Grand Master of English Freemasonry. In 1841 the Duke wrote to
Albert, Prince Consort (through the Prince Consort's household Treasurer) warmly commending the work of Harris, and stating that he had "experienced much advantage from his services in my library". ==Tracing boards==