A spring wind blows through a room, and the speaker lets it work on a shelf of old philosophy books. The image is simple: books left open to the air, losing their stale smell. But the poem treats it as something more. The wind is "hard at work," as if it were doing the reading, or the cleaning, on the speaker's behalf.
The most interesting line is the idea that the books are "nearer to being plain." Philosophy, with all its abstract argument, seems to shed some of its heaviness and become just paper, ink, and air, part of the ordinary physical world. The speaker seems quietly pleased by this, with a touch of humor about their own "musty" habits of thought.
The last two lines bring in a "bit of cold moon" shining in. It adds a clean, quiet light and a feeling of calm. The mood is fresh, light, and contemplative, as though the mind is being aired out along with the books.