[throwanem]

Forth, I believe it was, rather than APL, and specifically for the simple semantics and quick composition of a stack-based language. I recall being impressed with the author's rather detailed, but still likely accessible to non-wizards, description of the decision process and its outcome.

[abecedarius]

My favorite bit was summoning the Interpreter from Starting Forth -- you could recognize him from the description before it was explained. I mean, if you've read Starting Forth already.