Which is the best book for general section of niser entrance exam (NEST Exam) And for iiser entrance exam (IAT Exam)?? Please provide me n help me.

There is no particular syllabus prescribed for General Ability section. This section might consist random questions from History, Mathematics, Geograph, Science, English, General Knowledge and Current Affairs. For Mathematics, you need to concentrate on: Numbers, HCF and LCM of Numbers, Decimal Fractions, Square Roots and Cube Roots, Average, Problems on Ages, Percentage, Profit and Loss, Ratio and Proportion, Time and Work, Pipes and Cistern, Time and Distance, Problems on Trains, Boats and Streams, Alligation, Simple and Compound Interest, Logarithms, Area, Volume and Surface areas, Calendar, Clocks, Permutation and Combinations, Probability, Reasoning, Sitting Arrangements, Blood relations and Data Interpretation at Class 10 level. So you can stick with NCERT for this.

In English section, command on your English, Grammar, Vocabulary, Understanding of the given paragraph are of more importance. Reading English NewsPaper will also help you in your preparation.

Word Power Made Easy (by Norman Lewis)
This is the go-to book to improve your vocabulary. It is not a book to be read but a book to work with daily to enrich your vocabulary.

High School Grammar and Composition( by Wren and Martin)
It is a very good book for people who want to improve their grammar skills. It helps to understand and conceptualise different aspects of grammar and it is a must have book for anyone with poor grasp on verbal ability or who wants to make up with the basics of english.

For other topics, you can refer to NEST exam guide. News papers, Manorama Year book should help you for this section.