Book sales benefit from a large number of honest and diverse Amazon reviews.
The best way to get such reviews is to get current and new friends, associates and colleagues to review the book.
Here's an Amazon page that talks about getting Amazon book reviews. As with all book marketing advice, take it with a grain of salt. What worked last year might not work this year. What works for fiction might not work for nonfiction. What works for one genre of nonfiction might not work for another genre of nonfiction.
Get Your Book Reviewed on Amazon and Boost Your Sales