The theme of writing is interwoven with pretty much all of the themes of The House of Spirits – writing helps the characters "reclaim the past and overcome terrors" of their own (there's "Memory and the Past" and "Perseverance" for you). It's also primarily done by the female characters of the novel ("Women and Femininity") as a way of recording family history ("Family") and connecting to the spirits ("The Supernatural"). Most concretely, the novel itself is imagined as the product of Alba's time in prison, a victim of state violence (so that covers "Violence," "Freedom and Confinement," and "Politics"). And if you can figure out a way to relate writing to the theme of "Society and Class" too, we'll give you a cookie.
Despite the fact that Alba begins composing her testimony in solitary confinement, writing for her is never a completely solitary experience. She relies on the inspiration and support of the spirits, her companions in prison, and her living family members.
Writing, like all acts of creative expression found in the novel, from Alba's painting to Rosa's embroidery, is a solitary and introspective act that removes the author or artist from the people around her.