As usual, I'm second-guessing myself when I find myself disliking a book that has received awards, critical praise and a movie deal. But unfortunately I just could not enjoy this book.
There's an easy explanation for this: it was only after I was well into the novel, and started reading reviews in my perplexity, that I realized that everyone who manifestly enjoyed it claimed it was hilariously funny. I didn't find it so. There's a certain mild and steady amusement in the contrast, well maintained throughout the book, between the mannered, Victorian prose of the first-person narrator and the vicious, violent events he narrates (he and his brother, surname Sisters, are hired killers in gold-rush California), but it's not by any means hilarious, and it does not for me overcome the equally constant level of mild nausea at the violence. I rarely abandon a book, but I should have stopped with the description in the first few pages of the death of horses in a fire. In some ways, the human deaths (macabre though some of them were - think acid baths) were less upsetting than that.
I'm not planning to see the film, but I think it likely works if the story is treated as slapstick. The convention of slapstick is that you simply remove for yourself any obligation for empathy - the characters are obviously not real, so pain is not real, and you can laugh. Possibly those who enjoyed this book treated it in the same way, and it may be a failing in me as a reader that I was unable to do likewise. What inhibited me, to a large extent, was that there was clearly an attempt to make some sort of empathy happen for the narrator, who is the less brutal of two brothers, and who gives us a backstory to explain the psychopathy of his family. So, for me, between that and the horses (I don't think I've ever seen any film where the terrified, painful death of animals was slapstick and funny), I just couldn't get into the right frame of mind.
Possibly, also, I was spoiled for the historical setting by the fact that I just read an extremely rich evocation of the same period and place by Isabel Allende ("Daughter of Fortune"). In comparison, deWitt's use of the same basic facts about San Francisco and area in '49 seemed bald and uninterested.
That said, one thing saved this review from being a single star, and that is the control of and delight in the nineteenth-century language. DeWitt managed to create a unique voice here - whether he has others, or whether this is his default, other readers will have to tell me, because unfortunately I'm not likely to pursue the rest of his oeuvre.