How to select an XmlElement based on an attribute value?

Say I have the following xml

<?xml version="1.0"?>
<catalog>
   <book id="bk101">
      <author>Gambardella, Matthew</author>      
   </book>
   <book id="bk102">
      <author>Ralls, Kim</author>
   </book>
   <book id="bk103">
      <author>Corets, Eva</author>     
   </book>
</catalog>

I have an xmlNode representing the whole file which I read in from a file by doing

$myXml=New-Object XML
$myXml.Load("path to the books file")

How do I then select the book element which has the id value of "bk102" so I can then pass that Xmlnode off to another function expecting an XMlNode?

i.e my new node would be :

$Node = <book id="bk102">
       <author>Ralls, Kim</author>
    </book>

Thanks all. Been struggling all morning.