It consisted in 2 volumes. In 1086 William made a land register of his territory to know how much money. He could collect in terms of taxes and the results of this survey were written down in the Domesday Book, which is a valuable historical document regarding the social structure of England after the conquest of the Normans.
In order to be able to organize such a system/to achieve such a result the king had to record the names of the nobles who had given the lands, how much land they had been given, the value of the lands assigned to each noble. In this book he wanted to record all the data concerning/regarding the nobles who had been given the lands by him. He also ordered to record exactly how much the land was worth. He wanted to have clear in his mind the true value of each property assigned to each noble.
The reason why he wanted to collect these data lies in the fact that this system/such a procedure might help him to organize the country from the point of view of legal system, taxation, justice, legislation, administration.