The speaker plays at being an epic poet. "I sing the house fly" borrows the grand opening of old heroic poems, but the hero here is a small insect landing on a shelf in a library. That mismatch is gently funny, and it sets the tone: affectionate, a little mock-serious, and quietly observant.
The fly is the first to settle among the "cold books," and the repeated "books books books" makes the shelves feel endless, rows of silent, unmoving volumes. Against all that stillness, the fly is alive. The speaker compares it to a painted lord, which seems to mean a painted lady-type butterfly, so the humble fly is lifted to something delicate and decorative. The final line, "in the most precarious manner," makes the landing feel risky and lightly comic, as if the fly were balancing on the edge of something much bigger than it knows. Overall the mood is playful and tender, finding small life and wonder in an unlikely visitor.