Hi Rik,

You can use the starts-with() function for that - if you create a list like this where you link each letter to itself:

<ul>
    <li><a href="?letter=A">A</a></li>
    <li><a href="?letter=B">B</a></li>
    <li><a href="?letter=C">C</a></li>
    <!-- etc. -->
</ul>

Then you can grab the letter from the QueryString and show all the books starting with that letter, using something like this:

<!-- Grab the root of the site -->
<xsl:variable name="siteRoot" select="$currentPage/ancestor-or-self::*[@level = 1]" />

<!-- Grab the container for the books -->
<xsl:variable name="booksRoot" select="$siteRoot/Books" />

<xsl:template match="/">
    <xsl:variable name="letter" select="umbraco.library:RequestQueryString('letter')" />
    <xsl:apply-templates select="$booksRoot//Book[starts-with(@nodeName, $letter)]" />
</xsl:template>

<!-- Template for a Book -->
<xsl:template match="Book">
    <section class="book">
        <h1>
            <a href="umbraco.library:NiceUrl(@id)"><xsl:value-of select="@nodeName" /></a>
        </h1>
        <!-- etc. -->
    </section>
</xsl:template>

Note that you'll have to substitute the actual DocumentType aliases for Books and Book documents where they're used in the above code.

/Chriztian