Regards,
Mark

I forgot to mention the result I want: a new xml document with the
<Items> (and their children) from both files in a single list as
children of a new <List> element --that is, make a single <List> of both
files in a new document. Since I made my original tries a few years ago,
I no longer know what I did wrong then. Although my style sheet works, I
am confident I do not understand the proper way to do this.

You will need to decide whether you make one of the (two) files your
primary input document or whether you want to start with a named
template. If you choose one file as the primary input document then all
you need is

  <xsl:template match="/List">
    <xsl:copy>
      <xsl:copy-of select="Item, document('35Subjects.xml')/List/Item"/>
    </xsl:copy>
  </xsl:template>

  <xsl:template name="main">
     <List>
       <xsl:copy-of select="document('22Authors.xml'')/List/Item,
document('35Subjects.xml')/List/Item"/>
     </List>
  </xsl:template>

Martin Honnen
http://msmvps.com/blogs/martin_honnen/