Professor Norman F. Cantor (1929-2004) was a historian who specialized in medieval history.  He is best known for his introductory textbook on the middle ages and a large number of works of popular history.  

Imagining The Law: Common Law and the Foundations of the American Legal System (1997) is a tour de force on the English Common Law background of the modern American Legal System.  In fact, Cantor says that it would only take a few law school classes to get an Elizabethan barrister up to speed enough to be at home in a courtroom in late 20th century America.  As the Kirkus Review said when the book first came out :"(Cantor) persuasively argues that common law's roots are so deeply embedded in our culture that even a new Ice Age might not kill them.". 

For me, the most fascinating part of the book was the description of how the English Common Law was pretty much cobbled together out of necessity and expedience.  For instance, the Roman Civil Law System required a veritable army of full time trained judges.  The medieval kings of England desired to render justice on the cheap.  Just like many rural jurisdictions in the U.S. today still have a large number of part time judges for the same reason,  medieval England relied on local gentry to serve without pay as Justices of the Peace and relied upon a system of a small number of professional judges riding circuit.  Cantor details how no King or parliament actually created the jury of verdict, it just sort of rose up on its' own out of practice.  Originally jurymen would be members of the community who knew something about the dispute and he parties and could advise the royal justices as to who should prevail in a lawsuit.  Over time this developed into the jury of verdict.  

Written in a conversational style, Imagining the Law is a very informative and entertaining read for anyone interested in legal history.  Long out of print, this book is still worth a look.