The shell's part of a whole culture with its own history and literature. There's no one right way to learn about something like that, any more than there's one right way to approach human culture in general. Here're some suggestions for other things that might be helpful.
Perhaps unsurprisingly, Wikipedia has a lot of good information about command-line utilities. You could profitably start just about anywhere near their Unix entry. For example, this list of Unix commands.
There's a Unix & Linux Stack Exchange.
My edition of Linux in a Nutshell, by Ellen Siever et al., is a decade or more out of date, but I still flip through it from time to time and learn something new.
And last, I've been working for a while now on userland: a book about the command line for humans. If this guide just isn't doing the trick, userland's more literary and detailed take on the subject might be worth checking out. (On the downside, it doesn't have nearly as many animated GIFs.)
That's just about it for now, but stay tuned - from here we'll be tackling more advanced shell usage:
- An Illustrated Guide to Shell Magic: Standard I/O & Redirection, which covers features like pipes, redirection, and standard IO that allow us to stitch little commands together - the true power of the shell.
- Aliases, wildcards, loops, and other techniques for typing less and accomplishing more.
- Writing scripts to solve larger problems.
- What it means when we say that "everything is a file".
- System administration tasks like upgrading and installing software via
aptand other package managers.
- Data munging for fun and profit.