Nailed your problem.
You have no 'cover' declared. (naming the file does not do this)
<meta name="cover" content="cover"/>
<item href="cover.jpg" id="
cover" media-type="image/jpeg"/> ties the 2 lines together
<guide>
<reference type="text" title="" href="part0004.xhtml"/>
<reference type="toc" title="Contents" href="part0003.xhtml"/>
<reference href="titlepage.xhtml" title="Cover" type="cover"/>
</guide>