I am a C#, WPF developer and would like to know and understand the Microsoft UI Stack. I could not find much on the internet. I guess that this would be a huge topic, and might not be feasible to explain here, but any suggest online reading would be greatly appreciated.
I don't know what you mean by "Microsoft UI Stack", but I will try to answer.
Current development shows huge investment into XAML-based UI. WPF, Silverlight, WP and now WinRT support XAML-based UI. The main problem is, that while the bases are same, there are still some details, that makes transitioning from one implementation to another hard, or almost impossible. You can read more about that in The Manifold Blunders of Xaml. So even thought MS says that re-using XAML code between platforms will be simple, I though this is just wishfull thinking.
Windows Forms is "dead" now, where dead means that MS stopped any kind of support and are not planning to extend it in any way. I would highly discourage starting new project with WinForms.
Second thing is about web development. Here, it looks like MS is investing a lot into ASP.NET MVC instead of WebForms. But I'm not a web developer, so I cannot say more about this. It looks like WebForms is sharing same fate as WinForms.
There is also interesting move to HTML+JS for client-side application programming thanks to WinRT support. But I don't believe this is good alternative to XAML+C#. MS is only trying to look cool by supporting it, re-using resources they put into IE development and baiting developers who only programmed in JS, HTML and PHP.
I'm not sure what you mean when you say "Microsoft UI Stack". Since your question is pretty broad and general, I'd thought something general like Metro would be appropriate as an aswner.
Microsoft is pushing towards a new design style called Metro. The Windows Phone 7 UI is based on it, as is (part of) Windows 8. As you might have noticed, Microsoft is also updating its various site to use the new Metro style:
If you wan't to use the Metro style in your applications (platform independent), these links are interesting read and a good start. Some talk about Windows Phone, some about Windows 8, but they all cover general principles you can use on any platform.