In addition to the secondary sources listed below, there are rich resources within John Wesley's own work - particularly his journals and "The Characters of a Methodist." The General Rules were not static and it is possible to trace changes that were introduced over Wesley's time (and beyond).
Carter, Henry. The Methodist: A Study in Discipleship. 1st ed. Fellowship library. London: C.H. Kelly, 1914.
Davies, Rupert E.A History of the Methodist Church in Great Britain. London: Epworth P, 1965.
Nausner, Helmut. "The meaning of Wesley's general rules, an interpretation.." Asbury Theological Journal 44, no. 2 (Fall 1989): 43-60.
Watson, David Lowes. "Aldersgate Street and the General rules : the form and the power of Methodist discipleship.." In Aldersgate reconsidered, 33-47. Nashville: Kingswood Books, 1990.
"General Rules." The Encyclopedia of World Methodism. Nashville: United Methodist Pub. House, 1974.