I found the best way to understand Emacs Lisp (and Lisp in general) was to read the source code. The core Lisp interpreter is in 4 or 5 files that aren't huge.

I found the best way to understand Emacs Lisp (and Lisp in general) was to read the source code. The core Lisp interpreter is in 4 or 5 files that aren't huge. That was some years back so there might be more code now, of course. With that in hand, the actual Lisp code and built-in docs are good too.