What is Reddit's opinion of Unix in a Nutshell: A Desktop Quick Reference - Covers GNU/Linux, Mac OS X,and Solaris (In a Nutshell (O'Reilly))?

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The only thing that changes from one Linux distribution to another is the default software they chose to put into distribution.

You can turn your "Ubuntu" into any other distro, apart from something maybe very specific that it's in another distro, it's all about the software they're using.

Something that changes considerably among distributions is the package manager. Yum, pacman, dpkg, apt... This can be the difference between you being completely lost in how to use a distro.

Then, you have the desktop environment or the window manager. They're not the same thing, a Desktop Environment is a collection of "packages", it consist of a Window Manager, a File Manager, specific Libraries and so on, but you can use only a Window Manager and chose your other softwares, like the File Manager, if you'll be using any.

There are people that will use the OS only on the terminal, so a Window Manager or a Desktop Environment aren't necessary for some users and there are Distros that will reflect that, providing you only with a command line and if you want to start the X system you'll need to install a Window Manager (or a Desktop Environment) yourself.

See the difference between "distros" are just the software each person/group/company chooses to use.

Linux is the Kernel, the Kernel is responsible to provide the interaction between Software and Hardware, like memory management, I/O and connected devices.

You don't really want to learn "Linux", you want to learn Unix. You want to learn the Unix Filesystem and maybe the most famous Unix tools. Search for a UNIX book, or just download a free Unix manual, like https://9p.io/7thEdMan/bswv7.html.

Once you know your way around it, you might want to learn some Shell, so pick a shell (Bash, zsh, Fish, whatever) and learn about it, so you can learn your way around the Terminal.

What you really want, as I understand it, is to learn UNIX, so a book like those two:

https://www.amazon.com/Unix-in-a-Nutshell/dp/B0043D2DL4

https://www.amazon.com/Macintosh-Terminal-Pocket-Guide-Command-ebook/dp/B008CQA9TQ

This is what you want, even though the second one is for "mac", it's about learning Unix really, a Shell and the Terminal.

For why do you want to learn C remains a bit unclear, if you don't know why you want to learn it, any C book will do the trick.

If you want to learn real C..., you'll want to study Computer Architecture, you'll not do that in a few months, it's a "some years long" journey, and you'll have to learn other things in the way like Assembly, but again, I don't think that's what you want, you looks like someone that just want to learn the basics and to know how to operate the computer efficiently, pick the Unix book.