on Jane Friedman site:

Seeking blurbs—that is, quotes and endorsements—is a pre-publication task that most writers absolutely hate.

However, unless yours is a front-list title from a major publishing house (in which case the publisher may get the blurbs for you), securing those important words of praise is up to you, the author. Not your agent or editor or publicist. You.

That means you have to ask established authors—people you may not know, who may have no particular reason for wanting to help you—to spend a significant chunk of time reading your book, write nice things about it, and affix their names to it forever-and-ever.

It’s hard to imagine why anyone would say yes to such an audacious request, yet people do, all the time; hardly a book is issued nowadays that doesn’t include a quote or two. The challenge isn’t how to get authors to provide blurbs; it’s how to get them to blurb your book.