Itâ€™s currently a bit difficult to find good resources for Windows 8 App development.
Do you search for WinRT or Metro? Or even Windows Runtime?
Using XAML leads to WPF results or WP7 stuff.
Anyway here are few good resources:
Free consulting services
31 Days of Windows 8
Windows.UI.Xaml.Controls classes (WinRT)
UX guidelines for Store Apps
Some good Stackexchange Answers