Answer:
Crooked as a rainbow and slick as a plate ten thousand horses can't pull me straight.
I help you carry your books.
I always have to work with something in my eye.
I may seem real but it always turns out I was never there in the first place... you only see me during a certain resting stage.
You can see me in water, but I never get wet. What am I?
No matter how little or how much you use me, you change me every month.