In addition to the materials previously cited, the following sources might be useful:
BYU Organ Studies Web Page. Visit organ.byu.edu
Engel, James. An Introduction to Organ Registration. St. Louis, MO: Concordia, 1986.
Geer, E. Harold. Organ Registration in Theory and Practice. Glen Rock, NJ: J. Fischer, 1957. (Out of print.)
Goode, Jack C. Pipe Organ Registration. Nashville, TN: Abingdon Press, 1964. (Out of print.)
Lessons for the New Organist. (A set of thirty video lessons for the beginning organist.) Visit youtube.com and search "lessons for the new organist".
Leupold, Wayne. First Organ Book, Fourth Edition. Colfax, NC: Wayne Leupold Editions, Inc., 2009. Visit theleupoldfoundation.org
The New Ward Organist. (A set of eight video lessons for the beginning organist of the Church of Jesus Christ of Latter-day Saints.) Visit youtube.com and search "new ward organist".
Ritchie, George and George Stauffer. Organ Technique: Modern and Early. London: Oxford University Press, 2000.
Sumner, William Leslie. The Organ. 4th ed. London: MacDonald, 1973.
Williams, Peter. A New History of the Organ. Bloomington: Indiana University Press, 1980. (Out of print.)
Wolford, Darwin. Organ Studies for the Beginner. Delaware Water Gap, PA: Harold Flammer, 1978.
The American Organist. (The monthly magazine of the American Guild of Organists.) Visit American Guild of Organists
Diapason. (Bi-monthly organ journal.) Visit thediapason.com