The Enchanted Book item model has been modified so that CustomModelData values 1, 2, and 3 add more layers to the item texture.
Use the following commands and compare the given item to a regular Enchanted Book:
/give @s minecraft:enchanted_book{CustomModelData:1}
/give @s minecraft:enchanted_book{CustomModelData:2}
/give @s minecraft:enchanted_book{CustomModelData:3}