[kbob]

> What's at the heart of lisp that makes it so simple and elegant?

IMO, it's a synergy of several things. Trivial parsing*, homoiconicity, automatic memory management, dynamic types, first class symbols, first class everything†, extensible everything†, heavy reliance on anonymous functions, flexible scope rules.

All of those have since been adopted by other languages, but only Lisps have them all working together. (And not every Lisp has every feature.)

* ... until you consider various extensions

† almost

[kbob]

Oh, and I second the recommendation for Peter Norvig's Lispy. https://norvig.com/lispy.html

Or, if you want to do a low-level implementation, Quiennec's Lisp in Small Pieces.