I made a one-page bash guide a few years ago, which is what I consult when I forget things (like string variable operators). Near the bottom of the page is a short list of the best books and websites I've found on the subject.

http://www.adamponting.com/mac-terminal/

My goto: https://www.gnu.org/software/bash/manual/bashref.html

Create a script for something you're currently typing out by hand.. Then use those commands for a script.. Expand from there.

The small 1984 book, The Unix Programming Environment by Kernighan and Pike. Still the best written tutorial of shell programming I've found.