Thank you for feedback.
You may condider creating separate stylesheets
for Customer,InvoiceDetails, etc. to be applied
to the input document Invoice.xml. The useful reference which would be in line with the subject
of relational databases is "Building Oracle XML Applications" by Steve Muench from O'Reilly. Actually, chapter 7 on XSLT is available on web, see http://www.oreilly.com/catalog/orxmlapp/chapter/ch07.html#9820. I also would recommend "XSLT Quickly" by Bob DuCharme from Manning, see section 3.6 "Copying Elements to Result Tree". To extract Customer element from Invoice you may consider something like:
<xsl:template match="Customer">
<xsl:copy-of select=".">
</xsl:template>
Regards,
Igor Dayen