title for different bibliography entries

Hallo DocBook-users,

I am currently working on a bibliography. This bibliography contains 
several sources, books, online material, PDFs and so on. I need to 
seperate them by using a title, for example BOOKS, ONLINE MATERIAL and 
so on. I tried to use a empty <title/> element and a 
<subtitle>BOOKS</subtitle>. That works, but consequently these subtitles 
are included in the TOC. Are there any possibilities to create a 
bibliography with the requirements mentioned above? Here is an example 
how it should look like:

<!-- BOOKS title should start here-->
<biblioentry xml:id="ID01">
<abbrev>ABBREV</abbrev>
<author>
<personname>AUTHOR</personname>
</author>
<pubdate>2008</pubdate>
<title>TITLE</title>
</biblioentry>

<!-- ONLINE title should start here -->
<biblioentry xml:id="ID02">
<abbrev>ABBREV</abbrev>
<author>
<personname>AUTHOR</personname>
</author>
<pubdate>2008</pubdate>
<title>TITLE</title>
</biblioentry>

and so on.

I hope I verbalised quite clearly :-)

Thanks in advance,
Andreas