The poem sets two places against each other: the open, freezing outdoors and the enclosed work of a scriptorium, where texts are copied by hand. The speaker seems to be thinking about the price of that work. Out in the cold, where breathing feels like tasting the sky, a whole canon of literature might be had for far less than it costs to produce a few pages of ornate lettering on pale parchment.
The tone is wry but a little uneasy. "Life sentences" is a neat double meaning: sentences of writing, and sentences of imprisonment. It suggests the copyist gives up a whole life to the page. The final image, stones burning holes up through the snow, is stark and strange. Judging by the imagery, it hints at something hard and lasting pushing through the cold surface. The mood is austere, quiet, and slightly haunted, with a sense of both admiration and doubt about what this devotion costs.