Enchanting tweaks is a mod which allows you to customize vanilla enchanting mechanics. You can change item enchantability and the list of enchantments which can be applied on specific items (including items that cannot be enchanted by default). This offers modpack makers a new way to modify the game.
Besides that, this mod also improves some aspects of vanilla enchanting. You can now craft empty bookshelves ready to be filled with books. Not only does it look cool and allows you to easly change number of books close to enchanting table, but putting enchanted books there also increases your chance to get respective enchantments. It is also now possible to enchant shields and horse armor on enchanting table. And finally, this mod adds some new enchantments and modifies some vanilla ones as well.