Costume historians and re-enactors owe a lot to Janet Arnold's work. This book, with its in-depth examination of extant garments and patterns derived thereof, is a treasure for anyone's library. With many photos of real garments and paintings of similar fashions, one can examine closely how garments were constructed, their materials and embellishment.
The one weak point in this book is not due to Janet Arnold's research, it's due to the fact that so few extant garments from this time period remain. So we have some clothes from Norway, a few from England, etc...and so does not represent all of Europe's fashion. This is an important point, because the styles were very regional. If you are looking for patterns on only Venetian courtesans, or how people in Flanders dressed, for example, this would not be the book for you.
Nonetheless, if you want to know more about construction techniques, general cutting patterns, and exacting details, based on actual garments, this book is invaluable research material for historians and costumers alike.