Generally, a Doomsday Book is a land register where livestock and property are recorded. However, for a particular cult, it is a register required when one stands before the Lord in heaven on Doomsday, the Day of Judgment. All sins and good works are supposedly recorded on the book.
Ash Landers, the Queen's butler, goes to the Phantomhive manor. He informs Earl Ciel Phantomhive that outside of Preston, there is a deserted Catholic abbey that was burned down during the Reformation. A cult espousing deviant doctrines are gathering there. Its leader has the Doomsday Books of all followers. However, the books that they revere are used to record sins and good works of each individual, and it must be presented on Doomsday in front of the Lord in heaven.
There are rumors that the cult plans to revolt against the government, and the Queen laments that the people in Preston may be fearful of these heretics. Thus, the choice is left to Ciel whether to disperse them or to eradicate them.