"Recursive Functions of Symbolic Expressions and Their Computation by Machine, Part I" http://www-formal.stanford.edu/jmc/recursive.html Amazing that one can define an entire language in terms of only seven functions and two special forms: atom, car, cdr, cond, cons, eq, quote, lambda, and label.
I consider myself a pretty good software engineer, but I could never get my arms around LISP. I think I first tried it around 1964 on a PDP8 computer. It was fun to play with, but my boss at the Research Center kept asking how my "project" was coming along. I finally abandoned playing with LISP and got back the fun stuff I was getting paid to do.