Ever wonder about those effusive blurbs you see on book covers--you know, "This book is a must-have!" from Somebody Really Important. How do they get there, and what purpose do these endorsements serve?
Basically, it's an attempt to convince you to buy the book. If Professor J.D. Snickelfritz says this is the best book ever, then you'd be stupid if you didn't buy it, right? Surely Dr. Snickelfritz is unbiased and has no stake in whether you buy the book. Right?
The answer is "yes, most of the time." I don't suppose I know of anyone ever being paid to provide an endorsement, beyond getting a free copy of the book. But sometimes Dr. Snickelfritz might be a good friend of the author's. So he's not going to say the book stinks. And other times he might have his own book published by the same publisher, and has been prevailed upon by some well-meaning editor to give it a stamp of approval.
Most of the time, it works like this: The editor or publicist tells the author, "Hey, round up some of your high-profile friends and contact some of your idols and peers. Tell them we want to send them an advance copy of your book. If they like it, we'd like them to send us a nice quote we can use on the cover." Some authors are better at this than others. Sometimes it gets down to press time and we've got nothing. Other times we get a bountiful list of glowing reviews from famous and brainy people.
Now what about those customer reviews on Amazon.com? Those are all completely unbiased, right? Sadly, no. I think many authors put their friends up to writing positive reviews on Amazon. Maybe even some publishers (but not me--the worst I will admit to is clicking on the "This comment is not helpful" button when someone pans one of my books).