Hello!

I think it would be a good idea to have a general purpose source field. There is the URL field, which I would use if I found the recipe on the Internets. But where to put the information, that the recipe is from The Great Cooking Book, p. 362 or My best friend's Grandma? Right now there is no suitable field for that.

Having a look at https://schema.org/Recipe, maybe the https://schema.org/citation field would fit.