hi kunal,

Its named 'JEE- MAINS in 40 days' but it doesn't mean that you will get done with the study of whole JEE sylabus in 40 days.
It is a book for “revision” only and going through everything in a short period of time. Which is useful only when you're done with the entire syllabus before.
It is comprised only the last minute or revising stuff. That will help you go though the syllabus in less time. It also has numerical and MCQ in it which are quite helpful.
So basically it depends upon you and your preparation. That it is useful or not.If You Are Relying On this solely then it is not enough.Firstly You Should Learn and practice concepts using some good reference book and just solve this book in last few days.

I hope this answer will help, all the best for the future!!