“Every day I buckle on my guns and go out to patrol the dingy city. I’ve been doing it so long that I’m shaped to it, like a hand that’s been carrying buckets in the cold.” So begins author Marcel Theroux’s “Far North,” a novel of post-apocalypse set in Siberia. It’s an interesting geographic choice for this kind of story, as Siberia is one of the few places in the world that already looks as desolate and ravaged as a post-apocalyptic landscape. Theroux, who has both spent time on the Great Steppe, and also filmed a documentary on settlers who have chosen to move back to Chernobyl, does a remarkable job evoking the breath-freezing cold of that world, giving even the novel’s most implausible ideas the ring of truth.