Once you understand basic Lisp, (e.g., have worked the first half of the Little Lisper or the Little Schemer or the first few chapters of Structure and Interpretation of Computer Programs) the official Emacs Lisp manual [1], M-x describe-function and (progn (info "elisp") (Info-index topic)) are all you need. At least that has been my experience.
My favorite thing about writing Emacs Lisp code is that it is easy (e.g., just an 'apt get' on Debian) to install the aforementioned manual locally -- and the other resources I mentioned a part of the basic Emacs install -- and once I have installed that manual, I don't need internet connectivity when I am writing Emacs Lisp code. (Not having internet connectivity means I waste less time browsing around aimlessly.)
My favorite thing about writing Emacs Lisp code is that it is easy (e.g., just an 'apt get' on Debian) to install the aforementioned manual locally -- and the other resources I mentioned a part of the basic Emacs install -- and once I have installed that manual, I don't need internet connectivity when I am writing Emacs Lisp code. (Not having internet connectivity means I waste less time browsing around aimlessly.)
[1]: https://www.gnu.org/software/emacs/manual/elisp.html