Some of the Twilight's Hammer Cult cultists carry around little papers with gibberish written on them. To decipher them, a the book is used by their scribes to encode secret messages: the Twilight Lexicon. It's such an important tool that they keep its three chapters in different locations. The officers entrusted with the Lexicon are known as Twilight Keepers. One of them is the human Mayna. She is guarding the first chapter of the book in the camp in the far west of Silithus.
Drops [Twilight Lexicon - Chapter 1] for the quest  The Twilight Lexicon.