How to bind VB.NET WPF DataGrid and ListBox to display XML content?

<authors_table>
  <authors>
    <au_id>172-32-1176</au_id>
    <au_lname>White</au_lname>
    <au_fname>Johnson</au_fname>
    <phone>408 496-7223</phone>
    <address>10932 Bigge Rd.</address>
    <city>Menlo Park</city>
    <state>CA</state>
    <zip>94025</zip>
    <contract>true</contract>
  </authors>
  <authors>
    <au_id>213-46-8915</au_id>
    <au_lname>Green</au_lname>
    <au_fname>Margie</au_fname>
    <phone>415 986-7020</phone>
    <address>309 63rd St. #411</address>
    <city>Oakland</city>
    <state>CA</state>
    <zip>94618</zip>
    <contract>true</contract>
  </authors>
...

The following code does not work: I can display the correct information in the ListBox but when I select one id in that ListBox, the DataGrid is never populated with the expected data:

<window x:class="MainWindow" xmlns:x="#unknown">
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
Title="MainWindow" Height="350" Width="525">
<window.resources>
    <XmlDataProvider x:Key="XmlData" Source="E:\CMT\Automation\Testarea\Authors.xml"/>
</window.resources>
<grid>
    <listbox name="ListBox1" itemssource="{Binding Source={StaticResource XmlData}, XPath=Authors_Table/authors}" displaymemberpath="au_id" horizontalalignment="Left" height="202" margin="10,10,0,0" verticalalignment="Top" width="129" />
    <datagrid datacontext="{Binding ElementName=ListBox1, XPath=SelectedItem}" horizontalalignment="Left" margin="155,10,0,0" verticalalignment="Top" height="202" width="353">
        <datagrid.columns>
            <datagridtextcolumn x:name="FNameCol" binding="{Binding XPath=au_fname}" header="First Name" />
            <datagridtextcolumn x:name="LNameCol" binding="{Binding XPath=au_lname}" header="Last Name" />
            <datagridtextcolumn x:name="PhoneCol" binding="{Binding XPath=phone}" header="Phone" />
        </datagrid.columns>
    </datagrid>
</grid>


I obviously miss something in the way I bind the two WPF controls but I can't figure out what. If you have an idea, thanks in advance for your help.</window></authors_table>