DOCBOOK: Web servers in <bibliography>?

[Stephane Bortzmeyer <bortzmeyer at pasteur dot fr>]

I have a text with several references at the end. Some are purely Web servers, 
some are real books (and I put them in <bibliography>) and some are books with 
a Web server, like Norman Walsh's book about DocBook.

Is there a "current best practice" for Web servers (in <bibliography>? 
outside?) and for URLs of books? How can I rewrite the following:

	<bookbiblio>
	   <title>DocBook: The Definitive Guide</title>
	  <author>
	    <surname>Walsh</surname>
	    <firstname>Norman</firstname>
	  </author>
	  <author>
	    <surname>Muellner</surname>
	    <firstname>Leonard</firstname>
	  </author>
	  <editor>
	    <surname>O'Reilly</surname>
	  </editor>
	  <isbn>1-56592-580-7</isbn>
	  <abstract>
	    <para>I didn't read it yet...</para>
	  </abstract>
	</bookbiblio>

to add http://www.docbook.org/? <ulink> does not seem accepted in a 
<bookbiblio>, except in the <abstract>.