LINQ to XML vs. DOM
http://msdn.microsoft.com/en-us/library/bb387021.aspx
Fast way to convert XmlDocument into XDocument
Streaming Into LINQ to XML Using C# Custom Iterators and XmlReader
Note that XPath indexes are based on 1, not 0.
1.Basic XPath Expressions
http://www.liquid-technologies.com/xpath-tutorial.aspx
2.XmlDataProvider.XPath プロパティ
http://msdn.microsoft.com/ja-jp/library/system.windows.data.xmldataprovider.xpath.aspx
3.XPath Reference
http://www.w3schools.com/xpath/