In "To Build a Fire," how does London's style (particularly imagery) serve to establish and intensify the story's mood?
The tone is definitely a detached, matter-of-fact one. The narrator is clearly detached from the story, focusing on only the series of events, punctuated by terrific imagery/description.
London's narrator's detachment is important to the story because it allows the reader to focus on only the man and the dog and what is happening to them and around them. When reading this story, the reader does not focus on who the narrator is because London does such a marvelous job of keeping the focus on the man's journey to find the camp.
Despite the detached, matter-of-fact tone, the story is totally engrossing and keeps the reader on the edge of his or her seat! As things begin to look bleak for the protagonist, the reader cannot help but wonder if he will actually make it back to camp or simply die due to his lack of preparedness and inexperience with cold of this magnitude.
Early on, the story's tone might be described as austere. Look at the first line: " DAY HAD BROKEN COLD and grey, exceedingly cold and grey, when the man turned aside from the main Yukon trail and climbed the high earth bank…" This is clear, and even vivid, but the tone is distant. The focus is on the physical, and on extremes; this is shown through the repetitions, and simply through starting and staying there, rather than shifting to interpretation. Later, the style becomes more human, but still cold and distant. Consider this line: " A certain fear of death, dull and oppressive, came to him." That's death kept at an emotional distance. It's cool, like the land is cold.