When I say some stuff is optional, I'm not suggesting that you actually permanently skip it. Just that some stuff is more important than others. When you've covered the "required" work, you have enough knowledge that you could start to make your own programs. As an example, towards the back, there's a chapter on LINQ expressions. You do not have to know LINQ to make a C# application of any sort. (Well… unless you use a 3rd party library that forces you to use it.) Everything you can do with LINQ, you could do another way, with more primitive tools. LINQ is a better choice in some circumstances, but if it never existed, you could still write amazing software. (That's what people did before LINQ came out in C# 3.0.)
So some parts are not truly required, but it's hard to say you've mastered C# without having a pretty solid understanding of all of the material in the book. You could write your antivirus software without LINQ and be fine. (On the other hand, you probably do need a GUI of some sort, which I think is really one of your big points here. You technically could write it without a GUI (I wrote a security application that was purely command-line) but it's the kind of thing you really do want a GUI for.)
You said, "Console C# code is different from forms code"…. No. Not really. There's the part about Console.WriteLine, but literally everything else that you see and use is applicable in a GUI application. Variables, methods, classes, inheritance… you name it. It applies in a console app just as well as a GUI desktop app, or a game, or a service or a server. That's why I structured the book the way I did. My focus is on looking at the core C# language, the stuff anybody who does anything with C# will need to know. It's the starting point for everybody. The next step will be different, depending on what you want to do with it, but it's the same first step.
Do do GUI programming, you'll have to still learn a lot more, but everything you learn in this phase is still useful. Indeed, not only is it useful, it's the foundation that the various UI frameworks are built on.
Microsoft quit doing significant updates to Windows Forms when C# 3.0 came out and WPF superseded it. It's currently in "maintenance mode", which means they're still fixing bugs, and still supporting it, but they're not adding anything new to it anymore. That's not to say it's useless. WinForms is a bit easier to work with than WPF, but it's also less powerful and doesn't take advantage of the graphics device like WPF does.
In general, WPF and WinRT don't call things "forms" like WinForms does. They're UIElements, Controls, Windows, etc. The term "form" is only used on rare occasion, usually by a programmer who did a lot with WinForms before it.
To top it off, even WPF isn't getting updated at the moment. Microsoft switched to WinRT, which is sort of an evolution of WPF. WinRT still uses much of the same syntax (XAML) and design patterns that WPF has, but it is designed to be more cross platform, to target smart phones and other mobile devices, as well as desktops, laptops, and even the Xbox One. But developers haven't had much love for WinRT, and so the future is a bit uncertain right now.
It's not a horrible choice to use Windows Forms to start a new application right now, but I'd probably recommend WPF over WinForms in general. I always tell people it's twice as hard to learn, but ten times more powerful. (Both of those numbers are probably an exaggeration, but it gets the point across.)
If you've used VB.NET and WinForms, then going with C# and WinForms is actually a pretty good choice. If you've already used WinForms in another .NET language, then picking up WinForms in C# ought to be a piece of cake. (But then why not just stick with VB.NET? Why even switch languages? I prefer C#, but that's a personal preference, not a professional preference.)
As far as whether you should finish this book before moving on to a UI-specific book or just jump ahead to another book, that's up to you. It kind of depends on how much programming you've done in the past. Honestly, if you've got a year or two of VB.NET experience behind you, I'd start skimming through the book, rather than reading it. VB.NET and C# were both designed at the same time as the .NET Framework itself. Their syntax is quite different, but there's almost 1-to-1 parity between their features.
But otherwise, I'd say you should work your way though this book before tackling a UI book. You may be able to understand what the UI book is showing, but those books aren't going to be focused on explaining how the core language works. They're focused on how the UI framework works. I think it's worth reading it to pick up on those fundamentals that probably won't get explained or even appear in a UI book.
If you're antsy to get moving into the UI world, it doesn't have to take long. I think somebody with some programming experience and some free time could probably move through the book in just a few days. (For others, especially if they don't have much programming experience, it may take a few weeks or even a couple of months.) You could take a look at the little sections at the beginning of each chapter, and then jump ahead to the quizzes and Try It Out problems and make sure you didn't miss anything important.
But you know your situation a lot better than I do. If you feel like you'd rather just jump into a good WinForms or WPF book or something, then you're probably ready for it. This book would always be there as a fallback reference for you.