Someone already mentioned The art of command-line. It’s a start.

And just use it… you’ll get better at it with time and practice.

Also, make sure your use of the CLI enhances your workflow, or it won’t work.

Dedicate a set amount of time when you only use the cli to accomplish things. Pick simple, low risk things like cleaning up unneeded downloads in your downloads directory. Start with one file then try wildcards, brace expansion and regex.

View logs and grep to find specific events. Investigate (read only) what type of data is provided under different directories under /.

Use online resources to learn a scripting language, bash is convenient to start with, as it’s a common default shell and can be used for scripting. Learning bash can translate to one liners and eventually scripts.

This is a good resource, but I would recommend to not read it like a book, but maybe investigate sections of interest after you get a feel for some of the early topics: https://tldp.org/LDP/Bash-Beginners-Guide/html/