The most important thing to keep in mind about mediaeval cookery is that it can be as simple or as complex as you want it to be. The most popular class of complex cookery is generally known as subtleties. This category includes dishes made to resemble something else, as for instance a sweet that imitates a cooked fish; elaborate tableaux such as the Lenten favourite of fish dressed as pilgrims with eels as staves; or presentations such as roast swans re-covered in their feathers with necks and heads wired erect, made to breathe fire by means of spirit-soaked wads of fibre. Less popular in the SCA is the general mediaeval principle of balancing the humours in diet by reference to each food’s cold/heat and wetness/dryness. For instance, raw fruit was considered unhealthy because it was cold and wet, thereby leading to an excess of phlegm; eels, likewise, were cold and wet, but could be made safe by killing them in salt (hot and dry) and cooking them in wine; wild boar was hotter and drier than domestic pork, and so forth. Menus were devised on these principles and often tailored to the health concerns of the individual. Much mediaeval cooking, however, was as simple as a pease pottage or bread dipped in egg yolks and fried (the same as modern French toast).
     What you are likely to see at a local feast is somewhere between the two extremes. An ambitious cook may do a mid-level subtlety like a deboned quail inside a deboned duck inside a deboned chicken inside a deboned goose, or “hedgehogs” (little meat rolls studded with almond slivers). Savoury pies with meat or cheese, stews, and treats such as marzipan and sugared almonds are also quite common. Even fairly simple dishes can be and often are made more authentic and interesting by mediaeval sauces; a wide array of savoury and sweet sauces was essential to the mediaeval cook.
       The basic principles of SCA mediaeval cooking are: always restrict yourself to period ingredients or close equivalents (and reserve late-period/borderline New World foods like potatoes for appropriate occasions); do your best to choose recipes not only for their authenticity, but for how good they will taste to a modern eater (if you must do something weird to show off or educate, make sure there are simpler alternatives); and use redacted period recipes when you can.

Resources

Books
The best starting books I can recommend are
 The Medieval Kitchen, by Odile Redon, Francoise Sabban, and Silvano Serventi, tr. by Edward Schneider (many excellent recipes, from the amazingly simple to the truly complicated)
The Art of Medieval Cookery by Terence Scully (for the theories and methods behind medieval cooking)
The Medieval Cookbook by Maggie Black
Pleyn Delit-  Medieval Cookery for Modern Cooks by Constance Hieatt and Sharon Butler

Websites
SCA Potluck Recipes    http://www.advancenet.net/jscole/potlucktoc.htm
A very large resource - http://www.godecookery.com/scafeast/scafeast.htm

A listing of period cook books on line: http://moas.atlantia.sca.org/topics/cook.htm
Lastly a web based recipe swop : http://community.livejournal.com/sca_recipes