Three short lines, and the whole poem turns on a single comparison: a brown wing becomes a brown book. The title, "Bookworm," sets up the joke and the wonder together. A bookworm is someone who lives with their head down in pages, and here the speaker seems to have found something worth reading in the air instead.
The wing is probably a bird's, though the poem never says so. We only get its color and its shape, opening and closing like a cover. The dash gives a small pause, as if the speaker is still working out the comparison, and then the last line breaks in with a command: "look up." It reads like a quiet nudge to the reader, and perhaps to the speaker too, to lift their eyes from the page and notice what is flying overhead.
The mood is light, alert, and a little playful. There is a gentle delight in seeing nature as something to be read, and a suggestion that the best reading may happen outdoors.