What are the best sources to study and understand
- Process (reactions involved)
of Winkler's method and APHA method for CO2 estimation?
Ludwig (actually Lajos) Winkler, a chemist of hungarian heritage, published his method in 1888 in a German Journal: Ber. Dtsch. Chem. Ges., 1888, 21, 2843-2854.
Given that and the fact that Hungary was part of the Austro-Hungarian Empire at that time, it isn't overly astonishing that the publication is in German, rather than in English.
A nice description in English, with a discussion of technical obstacles, but without the math or equations, was published by Patrick G. McCormick in J. Chem. Educ., 1972, 49, 839-841 under the title The determination of dissolved oxygen by the Winkler method. A student laboratory experiment.