This big, but sadly quite overlooked site Fashion era is a goldmine with information about the history of fashion. Over 750 richly illustrated pages tell detailed info about how and why clothes of different eras where popular and what functions they could have, simply the site tells you all you need to know. From the ancient Egyptians most artistic feast-skirts to the hairfashion of ancient Greece to the modern european museums most daring creations and much much more. It's nuts how much info this site have, I just had to share it with you.
Tip, if you're intrested in armour and battledresses and such, look in the ancient costume directory, loads of good references.