The contents of The Forest of Darkening Glass are currently unknown to the general public. However, there has been some speculation that its contents will coincide with those described by Brandon Lachmann in The Glass Report.
Deirdre Green described finding The Forest of Darkening Glass in the Neithernor vault, among other books she remembered from her childhood, in her blog post “The Six Books.”
The Fourth Book.
A steep, spiraling ramp led to a watchtower. I remembered walking ahead of him, in case I slipped. I hadn’t been scared, but it was high, with no railing. He’d told me about the watchtower and I was determined to see the view from it. We read The Forest of Darkening Glass up there, twice. I wanted to make all new choices the second time and see how the story ended. I left it on the stone railing, with the telescope that had been there the last time I was.
“He’d given her a telescope to see the world beyond the castle. She had, up until then, never assumed they were anywhere other than New York. But now she saw the prismatic thing-shaped clouds, the great gray sea, the vast black forest at the base of the low, sharp mountains, and the tower sitting on top of it like a broken chimney. It all looked precious and endlessly fascinating. Like the maps you’d find on the inside cover of a fairytale. She looked and looked at the wild out there as his warm, comforting voice filled her ears, offering her safe or possibly perilous choices. He would laugh every time she made the same decision again. She couldn’t help it. She chose adventure and risk and exploration every time. Just like her father would’ve.”