My background is in Medieval History. I have always been fascinated with flavours and sounds in addition to kings and social movements. This, of course, led to the discovery of Fabulous Feasts.
Now as to the 100% accuracy of the recipes, I am unable to vouch. I can say, however, that they are all clearly discribed and easy to follow. I have a particular fondness for the arbolyetts and the parsley bread. I have cooked, at one time or another over the past 15 years, about 2/3 of the recipes -- some are definitely acquired tastes and some are pure show-stoppers.
Cosman also does a marvelous job of giving you food lore and manners of the latter half of the Middle Ages. From the salacious to the practical, all sorts of information is here. Her bibliography is helpful for those who want to follow further in her footsteps and her list of where to get the ingredients is good, but a bit dated. WARNING: at least one of the spice stores listed has shut since this book was first released.
This is a book for cooks and amateur historians. It is not a HISTORY book, but rather a cookbook that gets into a bit of history and allows you to sample the tastes of another time and place.