Minecraft wiki entry for Enchanting, associated text: Changing the enchantment levels offered by adding, removing, or blocking bookshelves alters the enchantments shown, but does not change possible enchantments; using another enchanting table with the previous bookshelf number still shows the previous enchantments. The enchantments for a particular enchantment level (with the same seed and item) do also differ depending on which row they appear in, but they are not "better" or "worse" based on the row despite the different resource costs.