Janis Common Lisp implementation is also very interesting. Moreover, it explains how to optimize CL code:

http://t-b-o-g.blogspot.com/2009/10/brians-brain-on-common-l...

http://t-b-o-g.blogspot.com/2009/10/brians-brain-on-common-l...

http://t-b-o-g.blogspot.com/2009/12/brians-brain-on-common-l...