Ans: For NSEC, you can simply thoroughly go through the NCERT. Besides, you can refer to these books:
Fundamentals of Analytical Chemistry, 2th Edition— Holt International
Analytical Chemistry, 5th Edition— John Wiley & Sons, Inc
Schaum's- series books
The abovementioned books are sufficient for INChO aspirants. These books have been designed to provide insight and intellect into almost all the areas of Chemistry asked in several Chemistry Olympiads. The books have been divided into chapters that include Atomic Structure, Biomolecules, Chemical Equilibrium, Chemical Energetics, Chemical Thermodynamics, Chemical Kinetics, Electrochemistry, Gaseous State, Redox Reactions, Radioactivity, & Organic Chemistry I and Organic Chemistry II, as per the syllabi of the INChO.