The Armourer and His Craft : From the XIth to the XVIth Century, by Charles ffoulkes
ISBN: 0486258513
Comments: I've had this one for awhile but haven't really looked at it yet

Book of Knighthood and Chivalry: With the Anonymous Ordene De Chevalerie, by Ramon Lull
ISBN: 1891448420
Comments: I've heard this refered to as "the squires handbook" Originaly writen in the 14th century, this is easily one of the most influential books on the ideas and ideals of chivalry ever writen

Chivalry, by Maurice Keen
ISBN: 0300033605
Comments: One of the best modern books on the subject of Chivalry I've found

The Craftsman's Handbook: "Il Libro dell' Arte", by Cennino d'Andrea Cennini
ISBN: 048620054X
Comments: A 15th century manual on the work of an Italian artist.

A History of Costume, by Carl Kohler
ISBN: 0486210308
Comments: This book is older than your mother. It's not the best costuming source around, though it does have some photograps of late-period extant garments, and pattern layouts, which (unfortunately) are iffy at best.

TECHNIQUES OF MEDIEVAL ARMOUR REPRODUCTION: THE 14TH CENTURY, by Brian R. Price
ISBN: 1581600984
Comments: Great hands on info about armour