Start with the title, because it is a slippery word. "Immanent" means dwelling within something, present all the way through it. It also sounds almost exactly like "imminent," meaning about to happen, and the poem seems to know it. The ague is "knocking," and the parts are "coming at night." Something is always on the threshold, and something is always already inside. The poem lives in the overlap.
Its central idea is that a self is not one sealed, tidy thing. It is spread across the things we make and the things that happen to us. The first line sounds like a quiet boast: "I am in all my books." Many writers hope to survive in their pages. But the next line adds an "and" that joins two very unlike things, books and sickness: "and I am the same as the ague." Ague is an old word for a fever with shivering fits, and its slightly antique feel suits a poem that begins among books. The grammar matters too. The speaker does not say "like the ague," which would be a simile. He says "the same as," which is a flat claim of identity.
That claim gets stranger in the third line, "knocking at my flesh." Knocking is what a visitor does at a door, someone outside asking to come in. So the speaker is both the house and the caller, the body being disturbed and the disturbance itself. The word "flesh" is blunt and physical, and set against "books" it suggests two bodies: the one made of paper and the one made of bone and nerve. The speaker is claiming both.
The second stanza turns from where the self lives to what has happened to it. "I am every interrupted part" is deliberately open. The parts could be unfinished sentences, broken sleep, the stop-and-start rhythm of a fever, or a life cut into pieces by illness. The poem never says, and that vagueness lets all these readings sit together. Whatever the parts are, they come "at night," the time of fevers, insomnia, dreams and solitary writing. Then the poem ends on "join," a verb with no object and no full stop. Join what? Join whom? The last sentence is itself interrupted, so the form acts out the "interrupted part" it describes.
The sound and shape of the poem are plain but careful. There is no rhyme, no punctuation, and no capital letter except the pronoun "I." That fits a voice that is tired, or confiding, or half asleep. "I am" opens the first, second and fourth lines like a quiet chant, a short litany of identity. The hard k sounds in "books," "ague" and "knocking" give the opening a rattle, so "knocking" sounds a little like what it names. The second line is the longest and runs on breathlessly before collapsing into the short, heavy "knocking at my flesh." The five lines split three and two, a full stanza followed by a smaller one, as if a fragment had been set beside the main piece. Even the layout feels partial and waiting to be completed.
This is what makes the poem worth reading. In very few words it takes a familiar hope, that a writer lives on in his books, and complicates it. The books are one place the self lives, but they are not the whole of it. The self is also the fever, the shaking, the broken night, and the pieces that refuse to stay put. Nothing is explained or consoled, and the poem trusts the reader to feel the unease of it. By stopping on "join," it leaves the question open. We cannot tell whether the scattered parts ever come together, or whether the reaching toward each other is all there is.