minecraft-assets/data/minecraft/recipes/book.json

{
  "type": "minecraft:crafting_shapeless",
  "ingredients": [
    {
      "item": "minecraft:paper"
    },
    {
      "item": "minecraft:paper"
    },
    {
      "item": "minecraft:paper"
    },
    {
      "item": "minecraft:leather"
    }
  ],
  "result": {
    "item": "minecraft:book"
  }
}