Hardback
Part of speech: adjective
Definition: Having a solid binding.
Part of speech: noun
Definition: A book with a solid binding.
Hardcover
Part of speech: adjective
Definition: Having a rigid binding.
Part of speech: noun
Definition: A book with a rigid binding, often of cardboard or leather.
Example sentence: The paperback is very interesting but I find it will never replace the hardcover book - it makes a very poor doorstop.