Hi grovelli,

$x is a variable within a flwor statement. You can begin to learn about flwor statements here: http://msdn.microsoft.com/en-us/library/ms190945.aspx

//EXPR is a path expression. A starting point again would be BOL here: http://msdn.microsoft.com/en-us/library/ms189919.aspx

With regard to a book regarding XML itself, there are a good couple of good SQL books pretty much dedicated to the XML data type and how to work with it e.g. XQuery, XML DML etc. I don't have a particular one to recommend as it depends on exactly what you want to learn, but the ones by Wrox or Apress are probably good starting points if you want to learn more about SQL XML.