Did Lewis leave out middle ground in saying that there are only two views that face all the facts? Certainly. But I think so intentionally.
Just as the Muslim divides the world into Muslim and non-Muslim, just as the Jew divides the world into Jew and Gentile, so too Lewis divides the world into and Christian worldview and a non-Christian one. Where I think he is wrong is in labeling the other worldview as dualism -- a world in which things are led by either a supernatural evil or a supernatural good. Sort of like the cartoons with an angel on one shoulder and a devil on the other. But then, he really isn't dividing the whole world up into these to camps, because by page 42 Lewis isn't at the beginning of his argument. He has progressed to the point in his argument where he is only talking about the theistic subset of the world.
So, given that, I'm not sure that he is wrong after all. In fact, isn't Islam rather dualistic? (I'm actually asking, not declaring.) I hear that in Islam life is a test. What sort of test? Well between submitting to Allah (i.e., good) and not submitting to Allah (i.e., evil). Certainly there are many different ways that one can be found to not submit to Allah, but in the end, as I have come to understand Islam as presented on these boards, life is either one or the other. And that is all that Lewis is saying when he describes all else as Dualism.
Before proceeding, I await comment and correction in case I have mis-spoken as regards putting Islam in the Dualism camp. I know that I did not consider everything that is true about Islam, and perhaps there is something in that which I did not consider that would be reason to say that I (and therefore Lewis) is wrong about this grouping.