The books of the same category should be together.
∴ 1 unit of art + 1 unit of fiction + 1 unit of adventure = 3 units can be permuted in 3! ways. 7 arts books can be permuted in 7! ways, 5 books on fiction can be permuted in 5! ways and 4 books on adventure can be done in 4! ways.
∴ Total number of ways is 3! × 7! × 5! × 4!.
