How good is the 40 days JEE Mains book ?

My advice would be not to fall for such marketing gimmicks. The book may be good, sure, but does it guarantee a good score in JEE Mains? I don't think so. You can buy the book, but don't fool yourself into believing you can make up for a lack of preparation for 2 years within 40 days. It is impossible and anybody who says otherwise is either lying or a complete charlatan