How to store an XML attribute in a frill register

I am looking at composing using XML as input. We have used classic mode for many years so I am familiar with XPP.
In classic mode I can store text by putting a <px;;#> in the Prestring and <pa> in the Poststring. I have no idea how to do a similar thing in XML mode.
This is a simple example of my XML:

<?xml version="1.0" encoding="utf-8"?>
<library state="PA">
  <books>
    <book>
      <title isbn="0-688-15428-X">Gung Ho!</title>
        <authors>
          <author>
            <name>
              <first-name>Ken</first-name>
              <last-name>Blanchard</last-name>
            </name>
          </author>
          <author>
            <name>
              <first-name>Sheldon</first-name>
              <last-name>Bowles</last-name>
            </name>
          </author>
        </authors>
      <publisher>
        <name>
          <corporation>William Morrow and Company, Inc.</corporation>
        </name>
        <address>
          <state>New York</state>
        </address>
      </publisher>
    </book>
  </books>
</library>

What I want to do is to store the “isbn” attribute value in a frill register to be used in a running header.
It must be easy but I just can’t find it in the documentation.
Any and all help will be appreciated.

Barry Frank
Reed Technology
Solaris & XPP 8.4C.1 SP #3