Clemence, Gerald M.; and Wollard, Edgar W.: Spherical Astronomy,
Academic Press, New York and London, 1966.
Web searches for "Celestial Coordinates" will turn up a number of interesting
web sites relating to this subject.
Today (27 Feb 2013), there are 2,850,000 results.
And when you come back a month later and search again, you get new web pages!