Doll Bones is a ghost story, so it's fitting that the narration is told from the ghostly remove of the third person. The narrator is invisible to readers—we never know who's talking—but from early on, it's clear this ghostly narrator is most interested in Zach's thoughts and feelings. Check this observation out:
That was why Zach loved playing: those moments where it seemed like he was accessing some other world, one that felt as real as anything. It was something he never wanted to give up. (1.8)
We don't get moments of insight like this one with the other two main characters, Alice and Poppy. Instead we can only infer things about them based on their actions and what Zach thinks. So while Zach might not tell his story in his own words, this is still definitely Zach's book.