Classical book of C.J. van Rijsbergen as Hypertext/CD-ROM version
http://www.dcs.gla.ac.uk/~iain/keith/

Recent trends in hierarchical document clustering: a critical review.
P. Willet.
Information Processing and Management. 1988. 577-597.

Clustering algorithms partition a set of objects into groups or clusters. The task of clustering (e.g. document clustering) is to create, or discover, a reasonable set of clusters for a given set of data types (e.g. documents). By contrast, the categorization task is to assign a new data type (e.g. a document) to one, or more, of a pre-existing set of classes (e.g. document classes).