Looking at expanding my Linux knowledge for my resume and would like to know what resources you guys have used

I'm definitely not bad with linux, but definitely nowhere near able to be a linux admin for a career.I'd like to work up to getting my RHCSA, and then possibly go higher.I have a beginner linux admin course on Udemy from Jason Cannon, but haven't gone too deep into it due to being focused on other things.I got a message that he's offering a few PDF copies of his linux books if I buy another of his courses for $15.Sounds like a good deal, but I won't bother if he's not a good resource.What have been your best linux resources?Thanks in advance!