CLOS
   Growing article, with links to many related topics. Wikipedia.
   http://en.wikipedia.org/wiki/CLOS

Cetus Links: CLOS
   Links to general Lisp information, including newsgroups and mailing lists.
   http://www.cetus-links.org/oo_clos.html

Implementation of DBC
   Document and source code on implementation of DBC for CLOS.
   http://www.muc.de/~hoelzl/tools/dbc/dbc.lisp

MetaObject Protocol: MOP
   CLOS MOP specification.
   http://www.alu.org/mop/

The Common Lisp Object System: An Overview
   Good introduction to motives for defining classes via generic functions. By Richard P. Gabriel, Linda DeMichiel; Lucid, Inc.
   http://www.dreamsongs.com/NewFiles/ECOOP.pdf