The Plague Tunnels is a tome found in Act V of Diablo III in the second level of the Plague Tunnels, or near the doorway at the end of the first level.
"The Repository of Bones, commonly called the Plague Tunnels, were originally used to bury Westmarch's indigent. However, when the Great Pestilence struck, the city decided the tunnels would serve just as well for plague victims, lest Westmarch become overwhelmed by the ever-increasing piles of the dead." - Abd al-Hazir