Question: What is difference in 'paperback' & 'hardcover' books except the physical difference of soft and hard cover? 

Answer: Apart from cover quality, paper quality, print size and layout may differ. Hardcover usually comes in stitched binding which is very durable compared to glued binding. Nowadays, there is an in-between stage called "trade paperback" which has larger print and somewhat better binding than mass market paperback, at usually 150-200% the price of mass market paperback.
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