The speaker begins with a odd claim: he is "in all" his books. The title, "Immanent," means existing within something, and that idea runs through the poem. Writing, it suggests, isn't separate from the person who made it. Pieces of the self live inside the pages.
Then the poem turns darker. The speaker says he is "the same as the ague," an old word for a fever that brings shivering and aching. So he is not only the thoughtful author but also the sickness shaking his body. Mind and illness, books and flesh, are all part of one self.
The last two lines feel eerie and a little tender. Every "interrupted part" of him arrives at night "to join," as if the scattered, unfinished pieces of a life gather together in the dark. The mood is quiet, restless and intimate, a mix of vulnerability and strange comfort at being made of so many fragments.