[mrfabbri]

Another interesting book on implementing Lisp (from a C perspective) is http://www.buildyourownlisp.com/.

Also, Lithp from fogus http://fogus.me/fun/lithp/ is a nice and brief exploration of implementing Lisp in Python and good example of literate programming.

[agumonkey]

Another interesting lisp is Maru http://www.reddit.com/r/programming/comments/157xdz/maru_is_...

from I. Piumarta (Inria, VPRI), sophisticated yet beautiful C code.

[mrfabbri]

1750 lines of fascinating C [encompassing an garbage collector and user-level implementations of user-defined types and structures, single-dispatch object-oriented "messaging", and generic "multimethods"]