Use the import/include two-step:
<?xml version="1.0" encoding="utf-8"?>
<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
version="1.0">
<xsl:import href="/sourceforge/docbook/xsl/html/docbook.xsl"/>
<xsl:include href="tpage.xsl"/>
</xsl:stylesheet>
Transform with that stylesheet.
The book is also available in HTML.