Here's a good article by William Leith about blurbing (presumably the one about the equally common promotional practice of getting your editor friends to commission pieces plugging your new book will follow soon).
Want to know a secret? See that book over there? The one that radio's Sandi Toksvig called "a thoroughly-entertaining rummage through the life and times of one of Britain's most eccentric businessmen?" She hadn't read it at the time. But she'd promised me a blurb, and when I offered her the manuscript, she freaked out, told me she was judging the Orange prize and reading an average of a dozen books a week, couldn't possibly take on mine as well, but was happy to put her name to anything I drafted.
"Not as good as The Littlest Viking" - Adam Macqueen