The official documentation[1] is quite good. An Introduction to Programming in Emacs Lisp[2] is just what it sounds like. It will get you familiar with writing Elisp, using the debugger, and some other basics. At that point, you should be able to navigate the Elisp Reference Manual without too much trouble.
Once you have that foundation, you shouldn't have much trouble scratching whatever itches that arise. The community is thriving so often a web search will return a blog or video explaining how to do exactly what you want. And once you're familiar with it, Elisp is a fairly readable language, so you can learn a lot from looking at the source for other packages to get ideas.
Edit: Sibling comment for Advent of Code is also a solid idea. Arguably Emacs's greatest strength is that it's a fantastic Elisp development environment.