Can I search for non-patent literature (XP) documents?

Espacenet is a patent database. You cannot use it to search for non-patent literature or specific XP numbers, at least not directly. However, if you know an XP number, you can search for citing patent documents by using the ct field identifier in the search field (smart search), i.e. ct = [XP number], or the Cited documents search field in Advanced search. Then, in the Citations, Cited documents view of each retrieved patent document, you will find information about the document registered under that XP number.