NomadBookItem calls ItemEntity#getAge on both the client and the dedicated server: https://github.com/Ladysnake/Nomad-Books/blob/master/src/main/java/ladysnake/nomadbooks/common/item/NomadBookItem.java#L376

                            return itemEntity.getAge() < 1;

However, the method does not exist on the dedicated server:

    @Environment(EnvType.CLIENT)
    public int getAge() {
        return this.age;
    }

Resulting in this crash

ive gotten the same thing a few times today. only occurs with my friends books but not my own.
crash-2020-04-07_00.15.48-server.txt