The bash book to rule them all
fabiensanglard.net/bash
What Jamie wrote
I don't have Efficient Linux at the Command Line but I love Sanglard's enthusiasm for the book. I followed the link to the Amazon product page and the reviews are incredible.
The command-line in Unix is an incredibly powerful place. I've shared my own delights at what you can do. This is a place that nearly all technologists can learn more, and the investment will pay off. If you are early in your tech career, get a book like this and really learn it. The shell is one area that you will get a career of value from knowing better. Related, this Full Bash Guide has a lot of info for free.