Create XmlDocument from subset of another

Hi all,

I'm trying to efficiently pull data out of an xml file into a XmlDocument
AND create another "sub" document based on one subtree of the document. For
example, say I've got:

<Books>
<Book>
<Title>Some book</Title>
</Book>
<Book>
<Title>Some other book</Title>
</Book>
<Books>

So far I've tried things like:

XmlDocument doc = new XmlDocument();
doc.LoadXml("Books>" +
"<Book><title>Some book</title></Book>" +
"<Book><title>Some other book</title></Book>"+
"</Books>");

XmlNode nodBook= doc.FirstChild.FirstChild;
XmlDocument docPart = new XmlDocument();
docPart.ImportNode( nodBook, true );


However, docPart only winds up as null. I've tried variations on the
ImportNode theme and received errors relating to the context being wrong for
the node.

Any pointers on create a sub-XmlDocument from an existing XmlDocument?

Thanks,

John