What has Carl Sagan written his books about?
SELECT DISTINCT ?uri WHERE { ?x <http://dbpedia.org/ontology/author> <http://dbpedia.org/resource/Carl_Sagan> . ?x <http://dbpedia.org/property/subject> ?uri  . ?x <http://www.w3.org/1999/02/22-rdf-syntax-ns#type> <http://dbpedia.org/ontology/Book>}
Carl_Sagan