Answer:
This is needed both for courage and hardcover books.
They can float and tickle, but their sound is rarely heard unless you're a pillow or bird. What are they?
It flies when it's on and floats coming off.
It's anything but amusing when this body part gets hit.
Two bodies have I, though both joined in one. The more still I stand, the quicker I run.
You use it between your head and your toes, the more it works the thinner it grows.