An example, just to make this a 15 seconds answer -
Say you have "styles.xaml" in a WPF library named "common" and you want to use it from your main application project:
<Application.Resources>
<ResourceDictionary>
<ResourceDictionary.MergedDictionaries>
<ResourceDictionary Source="pack://application:,,,/Common;component/styles.xaml"/>
</ResourceDictionary.MergedDictionaries>
</ResourceDictionary>
</Application.Resources>