Book of Secrets

The Book of Secrets is a Quest Item obtained from the Keeper of Secrets in the Throne of Secrets area, after completing Abaddon's Gate, the final mission of the Nightfall campaign. You can only get one per character upon mission completion but you can trade for more.

Take it to one of the Keepers behind the Keeper of Secrets (Keeper of Scythes, Bows etc) to swap it for a green weapon/item of your choice.

Collectors
Keeper of Shadows

Keeper of Steel Keeper of Elements Keeper of Nature Keeper of Light Keeper of Bone Keeper of Spirits Keeper of Illusion Keeper of Scythes Keeper of Spears??