If you want to improve your Command-Line/bash skills, "Efficient Linux at the Command Line" by @DanielJBarrett is a life changing read.
Really.
A mindblowing experience.
While you are at it, also check out offpunk :
yes, it is related.
Thank you so much @ploum for your kind words about my book "Efficient Linux at the Command Line" (published by @OReillyMedia)! I wrote this book to give beginning & intermediate #Linux users a path to become advanced command-line users. So glad to hear it's had an impact for you!