Tim Ferris did the same thing. Unfortunately, it works. Also, deleting negative reviews (even legitimate ones) is another tactic these aggressive marketers use. They also cross market and plug one another -I find his blog interesting - sometimes - but I have no interest in reading the book, which seems to be the usual advice packaged for under 30 professionals, which I am not.
I am one of the early reviewers. I reviewed the book at that time because I had received the book as a pre-order before the 23rd, and Ramit reached out to the readers on his blog who had pre-ordered to write a review. There was no obligation to do so, no reward, and no stipulation that anyone not be honest. If I felt the book had been trivial, or not helpful then I would have offered that info in the review. I understand some people may feel suspicious about the timing, but I call that smart marketing. It only works if the people who are reviewing the book found it to be compelling and worthy of recommending, which personally I did.
He was very recently featured on an online workshop area for creative types called Creative Live. thousands and thousands of people watch this resource. They show it live for free (often 3 days of workshops), then sell the recordings to those who want to rewatch. That could have easily influenced the increase of reviews.