The poem begins with a feeling of enclosure. Doors open onto more doors, and that endless nesting seems to turn the world dark. The title, "Closure," fits this idea: things shut, one after another, with no way through.
Then the speaker corrects themselves. Calling the world dark is "a fallacy," a mistake in thinking. Where they expected emptiness, they found "no void." Instead they found "a new writ," something freshly written, which suggests a message, a decree, or a beginning rather than an ending.
The last image is the strangest: "full as a book of glass." A glass book is something you can see into, fragile and bright, yet hard to read in the usual way. It makes the discovery feel both clear and delicate.
The mood moves from claustrophobic to quietly hopeful. The speaker doesn't shout about it. The tone is calm and a little surprised, as if relief has arrived by looking again at what seemed like a dead end.