Cauchy-Riemann equations and harmonic conjugates.
Program the numerical methods (like Runge-Kutta or Newton-Raphson) in Python or MATLAB to see how theoretical formulas translate into software code. To help tailor further recommendations, let me know:
Problems at the end of each chapter range from basic computational drills to highly challenging theoretical problems.
Purchase the 3rd or 4th Indian edition (paperback). It costs less than a pizza delivery and will serve you for 2–3 years of engineering math.