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

A Re-Examination of Text Categorization Methods.
Yiming Yang, Xin Liu.
Proceedings of SIGIR-99, 22nd ACM International Conference on Research and Development in Information Retrieval. 1999.

Learning to Classify Text Using Support Vector Machines.
Thorsten Joachims.
Kluwer Academic Publishers. Boston. 2002.

Machine Learning in Automated Text Categorization.
Fabrizio Sebastiani.
ACM Computing Surveys. 1999.

Special Issue on Automated Text Categorization.
T. Joachims and F. Sebastiani (editors).
Journal on Intelligent Information Systems. 2. 2002.

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). By contrast, 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).