sort me vash to price: 
<xsl:sort select="price" data-type="number"/>

sort me descending order: 
<xsl:sort select="price" data-type="number" order ="descending"/>

emfanizei ta proionta me price>10:
example:
<xsl: for-each select="//book">
<xsl:if test="price>10">
<tr>
.
.
.
</tr>
</xsl:if>

kanei tous titlous kokkinous:
<span style="color:#ff0000">
<xsl:value-of select= "title"/>
</span>

sort me vash ta category tous: 
<xsl:sort select="@*"/>