The fact that one of the most powerful financial institutions in the world maintains a "Doomsday Book" of emergency legal procedures, named after the historic record of English lands commissioned by William the Conqueror, suggests they take very seriously the possibility of catastrophic financial crises requiring extraordinary measures. The document states explicitly that "The Doomsday Book is not, and can never be a finished product" - implying an ongoing need to prepare for new types of financial emergencies.
This seems both novel and somewhat concerning, as it indicates the Federal Reserve anticipates and prepares for worst-case scenarios that could severely disrupt the financial system. The very existence and continuous updating of such a manual provides a window into how the Fed thinks about and prepares for potential crises.