Hello Mohit ,

As per your concern I'd like to inform you that you can always buy books from GK publication for NTPC exams. This book is somewhat more precise for NTPC instrumentation and control engineering recruitment exams. But you'll have to go thoroughly through this book for a clear understanding . Let it be quantitative aptitude problems or reasoning questions; language based errands or engineering chapters like BEE ; whatever that's required for this exam is mostly available there.

Hope my answer helps. All the best!