The book was sold for Rs. 198 and there was a 10% gain on the cost price. To calculate the cost price, the selling price (Rs. 198) is divided by the gain percentage (1.1 which is 100% + 10% gain) resulting in a cost price of Rs. 180. For a 15% gain, the cost price (Rs. 180) is multiplied by 115% (100% + 15% gain) resulting in a selling price of Rs. 207.