Adding more of them does not change the scale of cost fact. Once you have them you will want to upgrade them, then there's the fact that you need to upgrade or build more of the book part supply buildings. You will then need a bigger supply of the raw materials to supply those buildings. One more bookbinder building could mean up to about 20 more buildings to supply it. Depending on how you run your islands economy it could also badly impact on your troop production capacity.