In the first instance, meta.
Once the site enters the private beta stage, there will be an associated meta site where all scope issues can be discussed. Post a meta question asking "should non-fiction works be on-topic?" and, assuming you have some good arguments for why they should, self-answer with your reasoning. If that answer gets enough support, it becomes consensus - which means that if any question gets closed just for being about non-fiction, it will be reopened, by modhammer if necessary.
Also, lead by example.
Got some interesting questions about works of non-fiction literature? Post them! If they're well-received, they become a precedent, an example for future generations of Literature.SE users. The best reason for a given field to be on-topic is that it generates interesting questions which are relevant to the site. Meta consensuses are all very well for setting policies to be implemented, but for casual users, a lot of highly-voted questions about non-fiction literature speaks much louder. To quote a recent post by Community Manager Shog9 (emphasis mine):
you won't find an exhaustive list of allowed topics; just a few broad categories. If you want to know whether or not something is on-topic, you look at the actual questions: are My Little Pony questions generally closed and deleted, or upvoted and answered?