Answer:
I am needed for most animals and hardcover books. What am I?
I have keys but no locks. I have space but no room. You can enter but can't go outside. What am I?
The more you take away the bigger I get. What am I?
I am a path between high natural masses; remove the first letter to get a path between man-made masses.
I am two-faced but bear only one. I have no legs but travel widely. Men spill much blood over me. Kings leave their imprint on me. What am I?
It flows out of the soil, It burns you if it boils, And holds us in its coils, More valuable than gold, As black as it is old.