One aspect of Age of Iron's tone that really stands out is how intimate it feels. The entire novel is written as a long, extended letter from the narrator, Mrs. Curren, to her long-lost daughter. Mrs. Curren constantly refers to her daughter throughout the novel as "you." On one hand, when we read this, we sort of feel like we're intruding on some pretty private thoughts that we shouldn't even be reading. On the other hand, Mrs. Curren's repeated appeals to "you" make us, the readers, the "you" to whom she writes.
One advantage that this intimate tone gives Mrs. Curren is that it allows her to confess things that she might not necessarily express to people outside a close circle. She gets to express her worries, her fears, and her prejudices in an honest, open way because, as far as she knows, only her daughter will end up reading her words. She finds an outlet in which she can brood about her impending death, confess that her condition is worse than she originally let on, and try to make sense of everything that's going on around her.