I think the point is that Science Fiction and Fantasy generally have the largest overlapping audience. I like to think that means we're the more intelligent readers. Most likely it means we're a "niche" and only deserve so much shelf space. Either way, we're still the more intelligent audience.
So, I've pounded out about 2000 words (not much really) about BookB, mostly just getting some thoughts and information down so I don't lose it or confuse it. It occurred to me that writing Fantasy could be considerably MORE difficult than writing Science Fiction. I know what you may be thinking! OK, maybe I don't know what you may be thinking, but let me presume, I'm presumptuous like that. Anyway you may be thinking that Fantasy should be easier than writing Science Fiction, you can make up your own rules.
That right there is the problem.
With Science Fiction, you can fall back on science to assist you in making tough decisions: Anti-matter propulsion? Absolutely possible, but very dangerous! Great, I can build on that! Bio-engineering an all-in-one food-source? Absolutely possible as well, in fact it's likely being worked on to some degree right now. Flashy lasers and noisy space explosions? No, neither is really possible, you can't see lasers, you can't hear explosions that occur in space. See? Simple! Consider disbelief suspended by the power of science!
With Fantasy, Bloody Hell!!! I can create whatever I want really, but you have to still manage to suspend disbelief to some degree or you're going to fling the reader right out of your book and likely off of their sofa, where they'll get a nasty bruise on the underside of their elbow and curse your name, never reading one of your books again. That there is bad business for an aspiring writer. Which I apparently am, since I titled my blog to that effect.
I'm quite looking forward with glee to working on both BookA (Sci-Fi) and BookB (Fantasy) and the processes involved in these two very different and separate genres.