Encouraged by his father, McClellan made the most of his education only to find that the family could not afford to send him to university. His pacifist views featured strongly in his teenage and early adult years as did his love of science and practical experiments. He joined Tottenham Public Library as a junior assistant and progressed rapidly via Poplar and Penge to Chelmsford where his contribution to the home front was described as irreplaceable. After the war he returned to Tottenham where his pioneering approach he called Service in Depth was widely known as the Tottenham Experiment.. He combined fundamental ideas on purpose and function with practical means and empathy with staff to produce an outstanding service. The creation of the Greater London Council and the merging of Tottenham into the larger Borough of Haringey brought his practical application of his ideas to a premature end, but he was able to develop and write up his work at the College of Librarianship, Aberystwyth. His two books, The Reader, the Library and the Book, and The Logistics of a Public Library Bookstock, were published as a result.