Domesday Book, right?
Well, I'll assume you haven't and give you the very brief overview: it is the record of a survey of property in England back in 1086. Think property census on steroids.
Thanks to the Domesday Book we have a great glimpse into Medieval history. Unfortunately this book is pretty much in Latin and has a lot of shorthand and abbreviations. It isn't nearly as useful to the average GM as it could be.
There are some other awesome resources, however, that have used the Domesday Book, along with other historical documents. Using these resources some folks have written some great books....one that I'd recommend for any GM library if this was a post about GM libraries.
Using a small library of these book, S. John Ross wrote a good blog article entitled Medieval Demographics Made Easy. Distilling all this information a step further was Brandon Blackmoor, who made an awesome Kingdom & Town/City generator called The Domesday Book, which brings us back full circle from a historical and essentially unusable source document to a modern and totally awesome Free GM Resource.