For some bizarre reason, your XML document contains
another XML document embedded as a string within a node. Your
<cac:AdditionalItemProperty>
elements belong to this embedded document; you won't be able to query them from the parent document.
Unfortunately, the XML you've shown is invalid. It's not clear whether that's a result of you editing it for the question, or whether it's really malformed. But as it stands, the structure is:
<Invoice ...>
<ext:UBLExtensions>
<ext:UBLExtension>
<ext:ExtensionContent>
<sts:xxxExtensions>
<sts:InvoiceControl>
<sts:InvoiceAuthorization>
</sts:InvoiceAuthorization>
<sts:AuthorizationPeriod>
<cbc:StartDate>
</cbc:StartDate>
<cbc:EndDate>
</cbc:EndDate>
</sts:AuthorizationPeriod>
... CONTENT MISSING ...
<cac:AdditionalItemProperty>
<cbc:Name>
</cbc:Name>
<cbc:Value>
</cbc:Value>
</cac:AdditionalItemProperty>
<cac:AdditionalItemProperty>
<cbc:Name>
</cbc:Name>
<cbc:Value>
</cbc:Value>
</cac:AdditionalItemProperty>
<cac:AdditionalItemProperty>
<cbc:Name>
</cbc:Name>
<cbc:Value>
</cbc:Value>
<cbc:ValueQuantity>
</cbc:ValueQuantity>
</cac:AdditionalItemProperty>
</cac:Item>
<cac:Price>
<cbc:PriceAmount>
</cbc:PriceAmount>
<cbc:BaseQuantity>
</cbc:BaseQuantity>
</cac:Price>
</cac:InvoiceLine>
</Invoice>
Assuming that's just you editing the document for the question, and the
real XML is valid, you'll need to parse the embedded XML document to a new document before you can query it:
For Each Book As XmlNode In list
Dim bookXml As New XmlDocument()
bookXml.LoadXml(Book.InnerText.Trim())
Dim propertyNode As XmlNode = bookXml.SelectSingleNode("//cac:AdditionalItemProperty", manager)
...
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