I am currently in 1.20.4
resourcepacks\TEST Resource Pack\assets\minecraft\models\item\written-book.json:
{
"parent": "minecraft:item/generated",
"textures": {
"layer0": "minecraft:item/written_book",
"overrides": [
{ "predicate": {"custom_model_data": 1}, "model": "custom_model/leather-bound-book"}
}
}
I have tested with regular books and doesn't seem to work, the model and texture in-game is just the purple-black checkered box.