Most of what I have seen from museums are pavises, not regular shields. I have also seen pictures of some targes and a few bucklers. There are some good books on shields and the archeology behind them such as the Anglo Saxon shield and there is a really good book on shields that is written in German, but I don't remember the name of it off the top of my head.
Here is a good website with info on viking round shields.
http://members.ozemail.com.au/~chrisandpeter/shield/shield.html
If you are looking for books on how to fight with a shield, we don't have much information on how they fought with a shield in medieval times. We have the I.33 manual for medieval buckler use, but now known surving shield manual. We have examples of shield fighting with German dueling shields, iconagraphical evidence from paintins and statues, and we have your 16th century Italian Target manuals. That is about it.
Hope this helps.
Brian Hunt
GFS