XSLT将属性值求值为XPath？

<bookstore>
  <book category="fiction">
    <title>Harry Potter</title>
    <author>J.K. Rowling</author>
  </book>
  <book category="non-fiction">
    <title>Introduction to XSLT</title>
    <author>John Doe</author>
  </book>
</bookstore>

<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
  <xsl:template match="/">
    <xsl:apply-templates select="bookstore/book[@category='fiction']"/>
  </xsl:template>
  
  <xsl:template match="book">
    <xsl:value-of select="title"/>
    <xsl:text> by </xsl:text>
    <xsl:value-of select="author"/>
    <xsl:text>&#10;</xsl:text>
  </xsl:template>
</xsl:stylesheet>

Harry Potter by J.K. Rowling