John McCarthy. A Micro-Manual for Lisp: Not the Whole Truth. ACM SIGPLAN Notices 13, pp.215–216. 1978.

A “manual” in the sense of providing a complete implementation of Lisp – in Lisp itself.

Is that useful? A semantics expert would say “no”, since such a model has no real explanatory power: the recursion doesn’t bottom-out to define the concepts. However, a programming language expert might disagree and observe that this is meta-linguistic abstraction, the use of one high-level language to capture the definition of another. The “host” language provides a higher level of abstraction than is usual when writing interpreters and compilers, so it becomes easier to experiment with different forms of “target” language as there’s less low-level work to do. This benefit exists even when host and target are the same: essentially the target is bootstrapped and can then be adjusted while running within an earlier version of itself.

As far as I know this is the first example of meta-linguistic abstraction in the computer science literature. The idea was popularised by SICP (using Scheme), and forms the basis for a lot of modern bootstrapped compilers.