I’m talking about the knowledge graph concept google has recently incorporated to google search. But my question is wheter there is a knowledge graph that I can directly query via API or a client. For illustration, see this specific query:
search for "books" which contain the word "apple" and are linked to the "second war"
You may want to look at http://www.freebase.com. It’s a queriable entiry graph with a web API. I think it’s the biggest free knowledge base of this kind.