Hi,

You did not mentioned the class for which you are applying to NSTSE exam.  The NSTSE  (national level science talent search exam ) is conducted for the various level of school class. For class 1and 2 the paper include mathematics and general science subjects.

For class 3- 6 the paper include mathematics, general science and critical thinking Subjects.

From class 7 onward the subject included are -

Mathematics,Physics , Chemistry, Biology and Critical Thinking.

For mathematics, general science, biology, physics, chemisty the NCERt books are must as they will cover all the basic. Beside this here is the link for list of reference book for each class suggesed by the NTSE exam conducting authority - https://www.unifiedcouncil.com/reference_books.aspx