Some time ago, Protesilaos Stavrou published a nice book on Emacs Lisp. The idea is to bring a “big picture approach” to Elisp so that every Emacs user can experience the joy of fine tuning Emacs to meet their exact needs.
Just recently, Stavrou has added EPUB and PDF versions. In a way, it doesn’t matter since he provides the Org mode source and you can export that to almost any format you want. Now, though, he has nice PDF and EPUB versions that you can simply download and read in your preferred format.
It’s nice having the book available as, for example, an Info file but apparently I’m old fashioned and prefer to read it as a PDF. Others may like EPUB or Info. Whatever your preferred format, Stavrou has you covered.
There aren’t that many books addressing Elisp and how to use it. Marcin Borkowski’s Hacking your way around in Emacs is one good example and there are some short tutorials but Stavrou’s and Borkowski’s books are the only ones I can think of off hand that address Elisp exclusively.
I’ve skimmed through the book and it seems like a good introduction. If you’re an Emacs user and want to advance, you really should learn a bit of Elisp. It’s not as daunting as it might seem since even adjusting the Emacs configuration is an exercise in using Elisp.