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I can show you a example..

Assume the book price is 100/-

then sold for 10% loss that is 90 INR

again he buy the book with 10 loss ie; 9 INR

So total loss = 19 INR...

1st sell 100 * 90/100 = 90 INR loss 10 INR
2nd sell  90 * 110/100 = 99 INR loss 9 INR

All the game of loss and profit depends on cost pries
let the actual CP of book be =x
10 % of CP would be= 10/100 × x=10x/100
SP of book(on what pries B accept the book)= x-10x/100=90x/100 = CP of book for B
Now, CP of  book=90x/100
10 % of 90x/100=10/100 × 90x/100=900x/10000=9x/100
SP of book= 90x/100 + 9x/100=99x/100

Total loss to A means
initial CP of book for A=x
SP of book=99x/100