The Emacs Lisp Reference Manual is the obvious resource. It also comes with Emacs so once you are comfortable with the Info reader you can have it at your fingertips at all times. https://www.gnu.org/software/emacs/manual/elisp.html
Emacs also has comprehensive and high quality docstrings on functions and variables. I find them to be much more useful than in most other languages. You can very often get by just reading the docstrings.
And finally don't be afraid to look at the source. Bind find-function and find-variable to some convenient keys.
Start simple. Find a personal itch to scratch, write some interactive functions that you can bind to keys. Or a hook to make something work way you'd prefer.
Emacs also has comprehensive and high quality docstrings on functions and variables. I find them to be much more useful than in most other languages. You can very often get by just reading the docstrings.
And finally don't be afraid to look at the source. Bind find-function and find-variable to some convenient keys.
For those who are new to Lisp or want a refresher the Elisp Intro is great too: https://www.gnu.org/software/emacs/manual/eintr.html
Start simple. Find a personal itch to scratch, write some interactive functions that you can bind to keys. Or a hook to make something work way you'd prefer.