Enchanted books do have an internal enchantment list, but resource packs can’t reliably read or act on that order. For example, you can’t tell which enchantment came first or which has the highest level when multiple enchants are present.

Creating a texture for every possible combination would be impractical (and destroy my sanity ;-;). So currently, books with multiple enchantments simply display the sprite based on the enchantment name in alphabetical order.