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The Armorer's Cookbook 2 is one of the many Key Items found throughout Elden Ring, sold by the Coastal Nomadic Merchant in West Limgrave.
The Armorer's Cookbook 2 is a Key Item Recipe Book that will unlock extra crafting recipes once you have obtained it.
This includes the following recipes:
Purchased from the Nomadic Merchant on the West Limgrave beach along the northwest, not far from the Coastal Cave mini-dungeon. It is sold for 600 Runes.