Question: What non fiction subjects are covered in books by Doubleday publishers?
SPARQL:
SELECT DISTINCT ?uri WHERE { ?x <http://dbpedia.org/ontology/publisher> <http://dbpedia.org/resource/Doubleday_(publisher)> . ?x <http://dbpedia.org/ontology/nonFictionSubject> ?uri  . ?x <http://www.w3.org/1999/02/22-rdf-syntax-ns#type> <http://dbpedia.org/ontology/Book>}