A great way to learn the basics. It’ll be old, but that’s ok. It’s going to cover all the shell basics and then more. It’s still going to be useful, it’ll cost you pennies, you’ll be able to dip into it when you want, and you’ll be giving to a good cause.
Physical books are great. Internet goes out or other devices die and you need to complete work. These are reasons I like to have book references.
Also, one of my favorite things to do when reading is writing in my margins. When I figure something out or find something interesting I like to write it in my own words in the margins, and then if I have to reference again, I have my own words and explanation in the margins to help myself understand faster and better.
I also like to add sticky notes for the same purpose.
Find an online guide. Print to PDF or save as HTML/ODF/whatever you like. Annotate the document. Now notes and article are searchable. I guess a physical book might have an advantage if the power went out, but at that point you’re going to have other problems implementing the things the book suggests.
Internet service can go out without the power going out, you know. Not everyone has the good fortune to live in a place with quality infrastructure or multiple choices of ISP.
If the internet is gone I have no need for a Linux box.
E: and if it is really just me in my own LAN then I have the man pages