Patrick DeWitt's second novel, The Sisters Brothers, begins with a cruel image: a horse burning up in a fire, "his kicking, burning legs, his hot-popping eyeballs". If you don't care for that image, then you might not care for DeWitt's novel, especially as it is blurbed as "hilarious" and "relentlessly absorbing". The narrator and main character is Eli Sisters, a hired killer on the American west coast in 1851, around the time of the Gold Rush in the Sierra Nevada mountains. Eli's older brother is the dominant member of the killing team; while Eli is reflecting upon the death of his previous horse and the sad-sack qualities of his new horse, "Tub", Charlie is taking orders from the Commodore – they are to head down to San Francisco and put to death a mysterious stranger named Herman Kermit Warm for some undefined reason. The Sisters Brothers relates their odyssey and, like all odysseys, it is full of both strange adventures and revelations.

Eli and Charlie are in the habit of killing people, and they are surrounded by killers. Frontier life has always been cheap, and poor conditions created a zero-sum game, but since the discovery of gold there is even more incentive to kill and thereby get rich. As Eli follows Charlie, he begins to wonder whether there is a way out of this trap – he doesn't exactly feel guilt for the murders he has perpetrated, but he does feel a sort of spiritual fatigue, and certain people he meets along the way show him the possibilities for a more agreeable life: a dentist who introduces him to tooth brushing and a female accountant at a whorehouse who introduces him to intimate conversation. The index of Eli's developing humanity is his relationship to Tub; at one point he has the chance to swap the disappointing brute for a much nicer mount, and he does so, but then changes his mind because "he has been a faithful animal to me".

Eli and Charlie repeatedly discuss Eli's newfound qualms as they travel. Charlie is at first unconvinced, at least by moral arguments. He considers Eli's argument to be undermined by Eli's recent demotion to second banana, as well as by the fact that Eli has always tended to be a whiner. As children, after Charlie shot their abusive father and took their mother to have her broken arm set, he happened to leave Eli behind; the result was not predation by local carnivores, but critical sunburn – "Of course, just as soon as I left, you pulled your bonnet off." Eli, in other words, wasn't ever meant to be a killer in the first place, and at long last nature is asserting herself over nurture.

Everything has a cost in DeWitt's wild west, and once they get to San Francisco, steep inflation sets in. If Eli is thinking that assassination is a tough game, well, panning for gold is much more tedious and much less rewarding. There are more amenities, however – when they break into their quarry's hotel room, they discover an apparatus that allows them to talk to the front desk by means of a "large black horn emerging from the wall beside the bed". The Bay Area has always been the scene of rapid technological change.

A reader looking for meticulous depiction of Oregon and California in 1851, however, will have to look elsewhere. Eli barely gives the landscape a glance, and people met along the way are simple figures in his moral drama. Nor does Eli have any larger philosophical or sociohistorical insights to offer. His narrative style is flat and literal, which is perhaps supposed to be the hilarious part. Charlie's evident annoyance with him may be because Eli has never developed his sensibility beyond the mental age of about 13. Justice is done, however – at the end of The Sisters Brothers, even Eli can't help wondering if he should have been more careful what he asked for.

Sometimes, a novel is like a train: the first chapter is a comfortable seat in an attractive carriage,and the narrative speeds up. But there are other sorts of trains, and other sorts of novels. They rush by in the dark; passengers framed in the lighted windows are smiling and enjoying themselves. You aren't a passenger, you don't care about that destination, and the whole train rumbles on without you.

Jane Smiley's Private Life is published by Faber.