You see on lots of books little tribute to the book from say Time ,Mail and other papers .So do the printers hold back on the covers till the book critics of these papers read the book ?
Books are freely passed around reviewers long before they are published so that there is time to print covers with the reviews on. When i was at school, I had a friend whose Mum used to review all kinds of books.
Advanced review copies are usually sent out. Additionally the blurb is often compiled by another writer from the same stable, rather than the author on the cover :)
My friend was a book crtitic for the Manchester Evening News and he always got the books way before they were isssued to the public, ergo then I got to read them first too!!!