The Blackened Book is a branch of the Sharn Watch, chiefly concerned with investigating and containing threats to the city involving the use of magic. Most members of the Book are countermages, mages trained to monitor and dispel magical energy wielded by other mages. The Book are often called in to check crime scenes for residual magical energy, or to otherwise utilise their magical talents to solve crimes and apprehend magically-inclined criminals. Unlike the Sharn Watch as a whole, members of the Book are highly dedicated to their work and to the city, and aren't known for taking bribes.
They keep a record of all of the powerful spellcasters in Sharn, both for crime prevention purposes but also to leverage as a resource in times of crisis.
The headquarters of the Blackened Book is in the Warden Towers district of Middle Menthis, within the garrison of the Sharn Watch.