The three most important resources I've used:

- https://tldp.org/LDP/abs/html/

- https://mywiki.wooledge.org/BashGuide

- https://www.shellcheck.net/ (can tell you if there's a Bash-specific replacement for a POSIX mechanic)

I'll be reading this book over the break: https://fabiensanglard.net/bash/