The Book of Sent Soví
Joan Santanach (ed.)
Robin Vogelzang (trans.)
I'm torn. I don't do Spanish or Catalan cooking and can't read either language. What's more, there's already enough information available on medieval English and French cooking to keep me busy for the next hundred years or so.
On the other hand, the cuisines of England and France weren't completely isolated from the rest of Europe. There's considerable overlap with contemporary German and Italian sources, so there might be some interesting and useful bits of knowledge in this source. Further it's a source from the early 14th century, which means it could also provide insight into the cooking methods of that time period, spice consumption and availability, etc.
Dang, looks like I'm going to buy it.