"Writing GNU Emacs Extensions" by Bob Glickstein is a solid introduction. After that, the Emacs Lisp reference manual (available as "info elisp" in the shell) gives much more detail.

Inside Emacs, you can:

- "M-x apropos" to search for commands of interest

- "C-h f" to get documentation for particular functions by name

- "M-x find-function" to jump to the definition of a particular function

- "M-x find-library" to load a library into a buffer so you can read and (depending on your permissions) edit it.

The Elisp reference is very good, and most libraries are well documented. Between that, the ability to jump around the code base, and trying things out in the "scratch" buffer (it's an Elisp REPL disguised as an editing buffer), you should be able to get quite far.

Emacs has built in documentation. Learning emacs lisp is a good start:

https://www.gnu.org/software/emacs/manual/html_node/eintr/in...

This manual is also the part of the emacs distribution.