htmlhelp.hhc.folders.instead.books —
<xsl:param name="htmlhelp.hhc.folders.instead.books" select="1"></xsl:param>
This parameter controls whether there should be folder-like icons (1) or book-like icons (0) in ToC. If you want to use folder-like icons you must swith off binary ToC using htmlhelp.hhc.binary.