<xsl:variable name="doc1" select="document('doc1.xml')"/>
<xsl:key name="more-chapters" match="chapter" use="@id"/>

<xsl:template match="...">
  <xsl:for-each select="$document">
    <xsl:apply-templates select="key('more-chapters', ...)"/>
  <xsl:for-each>
</xsl:template>

Jarno

 XSL-List info and archive:  http://www.mulberrytech.com/xsl/xsl-list