Question: What are the non-fiction topics in the books whose authors is Terry Pratchett?
SPARQL:
SELECT DISTINCT ?uri WHERE { ?x <http://dbpedia.org/property/author> <http://dbpedia.org/resource/Terry_Pratchett> . ?x <http://dbpedia.org/ontology/nonFictionSubject> ?uri  . ?x <http://www.w3.org/1999/02/22-rdf-syntax-ns#type> <http://dbpedia.org/ontology/Book>}