I am looking for a medieval cookbook?

I doubt there is one. In medieval times, just about the only people who could read were monks and scribes - and I doubt that a single one of them was a woman. There was no need to make a cookbook or to write down recipes from that time because women were the cooks and the cooks were not the rich women who might be able to read well enough to run their home if necessary. All recipes form those times would have been ped down by word of mouth and any and all of the good recipes (not the fish jelly and stewed brains) ARE already in cookbooks, but because the dishes are still popular and we eat them all the time, you just don't recognize them as such. Besides, a lot of the cooking back them was simply roasting a large chunk of meat over the fire.