In just a few words, the speaker describes a strange, quiet sight: something in the water that looks like an open book. The next lines turn it into something else, a wing, and then add a darker detail. The ground "took" it "to eat." So what the speaker first sees as a book, a thing of pages and words, is also a dead or fallen winged creature being absorbed back into the earth.
The title, "Imago," is the word for the final adult stage of an insect, which helps explain the image. Judging by that, the wing may belong to something like a moth or butterfly, whose wings do look like open pages. The poem seems to link reading and decay, suggesting that books and bodies are both things that fade and get consumed.
The speaker doesn't say how they feel, which makes the tone hushed and observant. There's a gentle sadness here, but also calm, as if loss is simply part of how the world works.