If you're having trouble condensing your book into a few brief paragraphs, comb through your manuscript looking for passages to excerpt. This technique can be used very effectively if you have a strong paragraph that can be taken out of context and still make sense. Place the passage in quotes and underneath the quote identify the passage as having come from your book. Feature reviews in your copy. If you've been fortunate enough to have your book reviewed, include those reviews with your jacket copy.
People are curious about authors.
If you're writing a nonfiction book, your audience wants to know what establishes you as an authority on the subject. For works of fiction or poetry, your reader want to know where you've been previously published or a little about your background. The author bio is your chance. Don't be discouraged if writing promotional copy doesn't come naturally. Professional copywriters aren't as attached to a manuscript as the author, so take a lesson from the pros and look at your work as impartially as possible.