The title, "Cloistered," sets the scene: someone has shut themselves away, the way a monk or nun might. The speaker has chosen a small, enclosed space, and the poem's first line makes that choice clear. Yet the very place where they hide turns out to be where the night is "open." It's a nice reversal. Shutting the door doesn't close the world out; instead the darkness seems to reach in and stretch wide.
The phrase "that impossible hand" is the most mysterious part. It could be the night itself, reaching toward the speaker, or perhaps a hand on the page, a writer's or a reader's. The poem doesn't say, and that uncertainty is part of its pull. The final line, "I read and read and read," repeats itself like a habit or a compulsion. The speaker seems absorbed, maybe seeking comfort, maybe escaping something. The mood is quiet, solitary, and a little uncanny, with reading offering both shelter and a kind of obsessive hunger.