If I am going to pony up 20 bucks for a book, what do I care that a laundry list of brandnames praise it? Testimonials have become mere window dressing in an era in which we watch our pennies and are short on the time needed to read a book.
The value I am looking for is how the book helps me - directly. Does the point of view and how it's presented give me an edge in my professional life? Address a problem such as clinical depression I've struggled with in a provocative way? Provide an experience, such as living through someone's tragedy, I wouldn't have access to in other ways?
Those who provide the testimonials also cheapen their brandname. Soon enough the books without value will enter the realms of low-ranking hell. So much for their recommendation.
When books used to be published by the late Christopher Hitchens, no one cared about testimonials. All we wanted to know was what topic Hitchens was addressing and the angle he was taking.