Answer:
I come in different colors and shapes. Some parts of me are curvy some are straight. You can put me anywhere you like but there is only one right place for me.
I am the key that is the hardest to turn
I can be entertaining until you realize some pieces have been lost.
I am needed for most animals and hardcover books
I have no voice but I can teach you all there is to know. I have spines and hinges but I am not a door. Once I've told you all, I cannot tell you more. What am I?
Of these things - I have two. One for me - and one for you. And when you ask about the price, I simply smile and nod twice.