I have been looking around for a while but I haven't found a good resource teaching how to use C# specifically to build Windows apps with a GUI. Can you point me towards some good resources about this topic?
I would recommend http://www.wpftutorial.net/WPFIntroduction.html
WPF uses XAML to lay out the interface and a C# code-behind. It allows you to build interfaces for applications very easily and is a good starting point depending on the type of application you want to make.
I think you should first consider what technology for building GUI applications you want to use. Using C#, there are two main GUI development technologies: 1) Windows Forms 2) WPF (Windows Presentation Foundation)
With Windows Forms you can develop fast. But if you want to easily make application with beautiful user interface, very specific and fancy custom controls then WPF is better. But in case of WPF there are very often small things that can take forever. Its harder to debug. And... well, maybe its just better to start with Windows Forms.
For Windows Forms you can use for example this: www.java2s.com/Tutorial/CSharp/0460_GUI-Windows-Forms/Catalog0460_GUI-Windows-Forms.htm
For WPF this: http://www.wpftutorial.net/
And http://www.codeproject.com/ is just good for everything:D
Another free option, well you need to give them your contact information, but it costs no money is WPF Succinctly
You can try this site:
In Programming Environments - Microsoft Visual C#
and other tutorials with full apps.