Common Lisp Object System Specification (The Universe Known as LISP)

CLOS
The specification was published in the journal Lisp and Symbolic Computation (LASC) in Volume 1, Numbers 3-4, and was formally accepted by X3J13 in June 1988.
Overview
The introduction to that double issue.
Conceptual Material
Functional Material
Companion Description by Linda DeMichiel
It should be read by people unfamiliar with Lisp-based object-oriented languages or by people who do not wish to read the full specification.
CLOS: Integrating Object-Oriented and Functional Programming, 2004
CLOS in Context: The Shape of the Design Space, 2004