The BookMeta interface allows you to manipulate almost all NBT data of a book. Only the 'resolved' boolean tag is missing. 
I suggest adding 2 methods:

• boolean isResolved()
• void setResolved(boolean resolved)

These methods would give developers easy access to make books more powerful, as the default value for this tag is set to true in the implementation (opposing to vanilla: false).

I have also noticed that the resolved tag is not taken into consideration in places like

• org.bukkit.craftbukkit.v1_12_R1.inventory.CraftMetaBook#serialize
• org.bukkit.craftbukkit.v1_12_R1.inventory.CraftMetaBook#equalsCommon
• org.bukkit.craftbukkit.v1_12_R1.inventory.CraftMetaBook#applyHash