Easiest method is
<TD>
<xsl:value-of select="concat(//author/first-name,' ',//author/last-name)"/>
</TD>
when the XML Structure is
<title>The Confidence Man</title>
<author>
<first-name>Herman</first-name>
<last-name>Melville</last-name>
</author>
<price>11.99</price>