This is pretty self-explanitory, but to be clear it is a listing of the barons named in the Magna Charta of 1215 and some of their descendants who settled in America during the early colonial years.
It is a rather detailed account although not all dates are listed, not all names are listed, however it does give one a good idea of what the lineage is so that they can do further research.
A great asset to the book is that there are so many bibliographical references so that you will know WHERE to search further!

Probably one of the best genealogical indexes for the Middle Ages in print.