Lots of books on Amazon. Most on Linux, but that basically same as Mac. Search “command line”.
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Ha Ha, no, I was telegraphing mine. 
And, making a point: back in the day, there was only the command line; if you needed to use a computer, you needed to type commands after a blinking cursor, so we did, and it wasn’t all that scary. When my office migrated to PCs with hard drives (an astonishing 10 mg!) we graduated to Q-DOS* to manage our files. But up until then, it was the DOS prompt or nothing.
I’ll add that if you were used to working exclusively at a prompt from the keyboard, GUIs and mice seemed w-a-a-a-y too slow! How we’ve changed!
I’ll also add that I have reformatted the wrong drive while working in what we then rather uncharitably called WIMP mode (windows, icons, mice, and pointers …)
* Note: not this QDOS …
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To be truthful about relative age, I recall using punch cards in a computer lab a few of my undergraduate finance/business courses. That should hint at age. 
I don’t recall if I used DOS much. I do recall that my first personal computer was an early IBM and using Word Perfect and Lotus 1-2-3. 
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