I heard a radio programme recently in which an author was in the final stages of publication process. Part of what needed done was send advance copies to fellow writers who would write a positive blurb for use on the book cover. The writers had to guarantee to say something positive. In my naivety I always thought advance copies were sent to a number of writers and the most encouraging responses were selected for the cover blurb. But I'm pretty sure I heard a writer on the programme promise to write something positive before even seeing the book. I suppose appearing on the cover of someone else's book is free publicity and an endorsement of the blurb writer's credibility as someone whose opinion counts for something.