I have the following xml:

<book>
  <header>title</header>
  <a>text</a>
  <a>text</a>
  <otherelement>title</otherelement>
  <a>text</a>
  <a>text</a>
  <a/>
  <a>text</a>
</book>

I want to group each block of consecutive <a> tags inside an <al> tag,
like this:

<book>
  <header>title</header>
  <al>
    <a>text</a>
    <a>text</a>
  </al>
  <otherelement>title</otherelement>
  <al>
    <a>text</a>
    <a>text</a>
  </al>
  <al>
    <a>text</a>
  </al>
</book>

And like you can see in the example, I want to start a new <al> block
when an empty <a> tag is found. That's the hard part. Well, the other
part also isn't that easy...

I think you need a modified identity transform for this one.  The
following stylesheet walks the tree along the following-sibling axis,
allowing you to group <a>'s with text as you come across them:

<xsl:stylesheet version="2.0"
	xmlns:xsl="http://www.w3.org/1999/XSL/Transform">

<xsl:template match="node()">
	<xsl:copy>
		<xsl:copy-of select="@*"/>
		<xsl:apply-templates select="node()[1]"/>
	</xsl:copy>
	<xsl:apply-templates select="following-sibling::*[1]"/>
</xsl:template>

<xsl:template match="a[text()]">
	<al>
		<xsl:apply-templates select="." mode="fill"/>
	</al>
	<xsl:apply-templates select="following-sibling::*[not(self::a[text()])][1]"/>
</xsl:template>

<xsl:template match="a[text()]" mode="fill">
	<xsl:copy-of select="."/>
	<xsl:apply-templates
select="following-sibling::*[1][self::a[text()]]" mode="fill"/>
</xsl:template>

<book>
	<header>title</header>
	<al>
		<a>text</a>
		<a>text</a>
	</al>
	<otherelement>title</otherelement>
	<al>
		<a>text</a>
		<a>text</a>
	</al>
	<a/>
	<al>
		<a>text</a>
	</al>
</book>

Notice the <a/> which wasnt present in your example - easy to suppress
if that really is the case.

cheers
andrew