The speaker compares being loved badly to being a torn page from an old book. Someone who adores reading, and who handles other people's books with great gentleness, treated the speaker's heart carelessly. In response, the speaker does what the reader would understand best: writing letters.
The letters record small, painful details. The speaker notices how tender those hands are with someone else's novel, and how rough they were with the speaker's. They also remember the beloved crying out a name that wasn't theirs, then begging "her" to stay while telling the speaker to leave. By the end, the letter has been crumpled and discarded, and the speaker rescues it and stores it in the attic.
The mood is quiet, wounded, and tender, with a little bitterness. The speaker doesn't shout back. The final lines suggest a hope that writing might say what speech couldn't, even if no one reads it.