John Crowley's "Snow" is a commentary on how human memory works and, more specifically, a reasoning against the idea that human memory is somehow imperfect. This is not, of course, to say that Crowley is arguing that people are capable of remembering everything perfectly. That would be absurd. Instead, he's arguing that our inability, over time, to remember every detail of every moment of our lives is, in a sense, a good thing. This is because, instead of the minutiae of our lives, we are instead left with broad impressions of things. Perhaps the occasional, semi-conscious remembrance of a single person or event that affected us greatly. In the end, Crowley argues, and I wholeheartedly agree, that this second type of non-rational memory, the kind of memory that is composed of impressions and not "facts" or "details," is far more important to us.
Of course, there's a lot more going on here than this thesis on the nature of human memory. This just composes the backbone of a story the explores how people might try to overcome what is generally viewed as "imperfect" memory, and how their attempts, ultimately, fail. It's an intriguing idea, and very well fleshed out, although I still feel it's entirely secondary to the exploration of memory in general.