I've been doing some Xml related work lately and have found the following articles useful. I'm really posting them here so I don't lose them, but somebody else might get use of them too. Right? 
Best Practices for Representing XML in the .NET Framework
Design Guidelines for Working with XML in the .NET Framework
XmlNodes from XPathNodeIterator
Performant XML (I): Dynamic XPath expressions compilation
Performant XML (II): XPath execution tips