How can I separate differents enchantments from a book

There is no way to separate enchantments from a book in vanilla Minecraft. Some mods like Minefactory Reloaded offer ways to do it, but it can't be done in vanilla.

Should you decide to use the modded approach mentioned by Unionhawk, MineFactory Reloaded offers a machine block that can be used to separate enchantments from a book.

From the Feed The Beast wiki:

The Auto-Disenchanter is a machine added by MineFactory Reloaded which will extract a randomly chosen enchantment from an enchanted item and store it in a Book, turning it into an Enchanted Book. To operate, it requires you to insert an enchanted item and a book, along with a supply of power. When an item has been disenchanted, it will take some damage, which can break the item if it was already sufficiently damaged.

I had a Punch I, Infinity, and feather Falling IV and I used it on my bow thinking my book will give me back the unused one, but no. It does not give your Enchantments back. Try using the jumbled enchanted books on important stuff like armor and weaponry. That's all.