Metaobject protocols: Why we want them and what else they can do (1993) [pdf]

Absolutely seminal paper. In 1997 Alan Kay called the book by the same authors, The Art of the Metaobject Protocol: "the best book anybody's written in ten years."

https://en.wikipedia.org/wiki/The_Art_of_the_Metaobject_Prot...

I strongly feel that if you don't know MOP and CLOS, you shouldn't be allowed to call yourself an OO expert.

EDIT: And how much I do miss when Dr. Dobbs was actually good:

http://www.drdobbs.com/parallel/lisp-classes-in-the-metaobje...