The math in "Theory of Wing Sections" is definitely challenging. While it is of historical interest, I wouldn't spend too much time trying to understand it if your interests are practical. Before the advent of modern computers, aerodynamicists were forced to "shoehorn" problems into constructs that were tractable either analytically or using the limited computing power available. Many early methods (such as those in ToWS) became obsolete as computing power allowed problems to be attacked with fewer simplifying assumptions.