Thanks for all the help.
PS : Apparently, instead of calculating the heat absorbed, my book calculated the heat released, and then mistakenly put it in the efficiency formula, which resulted in a wrong answer.
Here is what I did :
work done in going from A to C,
W1 = 2nRToln(2) (isothermal process)
work done in going from C to B,
W1 = pΔV = nRΔT = -nRTo (isobaric process)
work done in going from B to A,
W3 = 0 (isochoric process)
so, total work done = W1 + W2 + W3...