HOW A PIPE ORGAN WORKS

Organ History:  the Pipe Organ from its Origin through the 20th century - panther.bsc.edu/~jhcook/OrgHist/  An extensive site, including detailed explanations of how the organ works with animated diagrams and video clips.
The Encyclopedia of Organ Stops - http://www.organstops.org/
The Young Person's Guide to the Pipe Organ - suitable for all ages - www.agohq.org/guide/