The Asilomar Institute for Information Architecture (AIFIA) has republished this article by Dr. Louise Spiteri from 1998. Don't be fooled by the title of this "simplified" explanation of facet analysis and classification. Clearly a difficult topic this is a "meaty" introduction to faceted classification and an excellent resource.

S. R. Ranganathan and the Classification Research Group (CRG). Ranganathan developed the theory of facet analysis because he was dissatisfied with the inability of traditional enumerative bibliographic classification systems to allow for the expression of compound subjects (Ranganathan 1962, 1967). Classification systems such as the Dewey Decimal Classification (DDC) and the Library of Congress Classification (LCC) attempt to enumerate topics expressed in published works. Such enumerative systems do not allow easily for the combination of terms from different parts of the classification schedules to express compound subjects. (from introduction)

Link:
http://aifia.org/pg/a_simplified_model_for_facet_analysis.php