The Cult of Sigmar (WAR History and Lore)  

This unlock is located inside the RVR lake, so if you happen to be over-leveled for the area and are coming back to complete your unlocks, prepare to be poultry.

From the edge of the RVR area, but still inside the boundaries of Troll Country, at coordinates 10k/52k you can see the abandoned shrine, braziers still alight. Approach the shrine to trigger the tome unlock.

History & Lore in Troll Country