The repository spans from 1984 to 1990.
The group also developed a portable CLOS implementation (PCL, Portable Common Loops) to experiment and verify their design with feedback from the community:
Flavors and LOOPS were developed around 1980 and were the direct inspirations. The AMOP book was then published 1991.
AMOP is a very good and extremely well written book about the CLOS Meta Object Protocol.
It also is fortunately / unfortunately one of the books with the best Lisp programming style. 'fortunately' because it is really great, 'unfortunately' the great programming style is tied to a slightly esoteric topic - object-oriented meta-object protocols. ;-)
Whatever motivation you have, a) reading a book with great Lisp style or b) learning about the CLOS MOP, the book is highly recommended.