See https://gerrit.wikimedia.org/r/163139 (and T31648).
https://en.wikipedia.org/w/index.php?title=Special:Book&bookcmd=render_article&arttitle=EBay&oldid=651707381&writer=rdf2latex demonstrates that we are not observing the DISPLAYTITLE magic word. This shows up in Parsoid output as:
<meta property="mw:PageProp/displaytitle" content="eBay" about="#mwt4"/>
For subpages,
https://it.wikisource.org/w/index.php?title=Speciale:Libro&bookcmd=rendering&return_to=Fiore+di+virt%C3%B9%2FI&collection_id=e0c6942ac5b1e95c718bbf040889e82f314bad8e&writer=rdf2latex
demonstrates that we use the full page title. We should probably use the subpage title instead (unless this is a single-page book?).