It depends on how you define “kosher”, and perhaps is ultimately a question for whomever you rely on for halachic decision making.
In the case of literature there are both issues of decency (what we don’t talk about on YWN) and also of apikorses. Many stories such as Moby Dick, Harry Potter, The Chronicles of Narnia, etc., all are considered to have “Christian” themes. For non-fiction what is “kosher” is much more complicated since you read non-fiction not for entertainment but to gather facts to make judgments pertaining to the real world.
Perhaps it would be useful if there was a site that review non-Jewish literature from a Torah perspective (and written by someone who if familiar enough with the goyim’s culture to realize a religious perspective).