Answer:
I begin your sentences. What am I?
I have a strong affection for belly buttons.
I can go through glass without breaking it. What am I?
Sing me to a baby about to go to sleep.
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?
Whoever made me don't want me. Whoever bought me don't need me. Whoever use me don't know me.