Really, you should read the book. It's very good.
Seriously, it was nominated for all kinds of awards.
What I will tell you below will probably significantly diminish your enjoyment of the book. There are plenty of other things to love about the book besides this one surprise, of course, as Bull is a superb writer, but this is a big and important revelation with major plot implications.
Even if you patted yourself on the back for figuring out "The Sixth Sense" halfway through, this one is a tough one to figure out. The secret is hidden in such a way that it's not at all obvious that there even is a secret. Even if I were to tell you that the book's narrator is somewhat unreliable (though not intentionally), you probably won't figure it out, because it's not something that character says, but rather something that goes completely unsaid -- without you ever noticing.
Okay, here goes. This is your last chance to bail out.
Sparrow, the narrator and protagonist of Emma Bull's Bone Dance, has a big secret, which Bull has cleverly concealed by writing the novel in the first person.
Usually writers write stories in the first person to give the reader an impression of intimacy, of actually knowing the narrator's very thoughts. A limitation of first-person viewpoint is that you can't show what people are doing when the narrator isn't there. It is a difficult viewpoint to use skillfully and easy to use badly. Bull uses both the strengths and the limitations of first-person viewpoint to hide Sparrow's secret from the reader.
The key to Sparrow's secret is that the first-person singular pronoun is gender-neutral. You can write "I did this" and "I did that" all day without disclosing whether you are male or female. In this way, the first-person viewpoint allows the writer to tell the story without ever revealing the narrator's gender.
This characteristic is vital to the narrative structure of Bone Dance because Sparrow doesn't have a gender. Sparrow is completely neuter.
The apparent intimacy of the first-person narration (we are, after all, supposedly inside Sparrow's head) allows Bull to convince us, seemingly without effort, that we know everything important about the character. But Sparrow just doesn't ever seem to have occasion to think about the ol' dangly bits. Poor Sparrow, we think, too busy surviving in the post-Apocalyptic landscape to ever get laid! And conveniently, we're used to bathroom functions being edited out of fiction. Furthermore, because we can only see the action through Sparrow's eyes, it doesn't strike us as odd that we never witness other characters referring to Sparrow using "he" or "she."
The character's name was also clearly chosen to be ambiguous. "Sparrow" works better than familiar unisex names like "Pat" or "Leslie," as these are well-known enough to clue readers in that there is something odd about Sparrow's gender. SF readers, however, are quite accepting of neo-tribal names like "Sparrow." (Bull knows her audience very well.)
Sparrow's secret would have been extremely difficult to conceal had the novel been written in the third person. And yet, we have read so many novels written in the first person that we don't even notice what we're not being told about Sparrow. Most readers apparently decided fairly early on that Sparrow was male, due to "his" interest in technology and old media, and didn't give the matter a second thought until the revelation.
Had we known from the beginning of the novel that there was something unusual about Sparrow, the revelation of what "he" really was would have had far less impact. Worse, if we were actively trying to figure out Sparrow's secret, the solution to the puzzle might have let us down when it was finally revealed.
But we don't know there is anything unusual about Sparrow. Since we see the story's world through Sparrow's eyes, we have no way of knowing how unusual "he" is. Are the people Sparrow hangs out with representative of the population of "his" world, or are they just the kinds of people Sparrow likes to hang out with? Selection bias, anyone?
Because of "his" secret, Sparrow never forms close relationships with other characters, which means we never really get a clear enough glimpse of them to know whether they think Sparrow is in any way unusual. Yet even this cool distance doesn't raise the reader's suspicions -- after the Apocalypse, who wouldn't be guarded in their relationships with others?
There are lots of stories (SF and otherwise) with unreliable narrators, but Bull is the only writer I know of to have gone so far as to conceal the narrator's gender, or lack thereof -- though I'll admit I simply might not have heard of other writers who have managed it. It's a good trick, but it's the sort of trick that would be difficult to use again now that Bull has done it so well.
By the way, Sparrow is neuter because... well, this is a further spoiler. But we've come this far. In any case, I can couch it in terms you won't understand until you're well into the book: Sparrow is a chevaux grown in a vat to be "ridden" by the Horsemen.
As I said, read the book. It's genius.