AUTHOR:
<xsl:for-each select="AUTHOR">
<xsl:apply-templates select="."/>
<xsl:if test="position() != last()">,</xsl:if>
</xsl:for-each>
Alternatively you can put the same <xsl:if> instruction inside the match="AUTHOR" template.
Michael Kay
http://www.saxonica.com/
Author, XSLT Programmer's Reference and XPath 2.0 Programmer's Reference