on all the Skill Big Books, we are still missing a recipe for the Potion and Engeneering Big books. I would suggest to create them. Also i think An big book of Alchemy would fullfill the mixing Skill Big Books.
Before anyone say but Potions and Alchemy are already so easy to level. I agree. I actually do not think that to many people will read Big Potion Books after they reached a certain level. Or would need to read an Alchemy Book at all. But I think those books should exist and be makeable for the very pro of the Craft. Simply as a Sign of Hey you are a pro.