The LispWorks manuals do not attempt to describe Lisp itself. For definitive information on Common Lisp, including CLOS, consult the American National Standard X3.226 for Common Lisp. An HTML version of this document is supplied with LispWorks and can be accessed from the Help menu.
For information on CLOS, Sonya E. Keene's book Object-Oriented Programming in Common Lisp: A Programmers' Guide is very helpful. This book is published by Addison-Wesley.
For an account of Metaobject protocols as well as a detailed study of an implementation of CLOS see Kiczales, Rivieres and Bobrow, The Art of the Meta-Object Protocol , published by MIT Press. The LispWorks MOP mostly conforms to the MOP described in chapters 5 & 6 of this book.