Many authors will tell you, including myself, that book blurbs are harder to write than the book itself! After creating numerous characters, devising a complex plot, and describing the time and place over hundreds of pages, distilling this down into a few sentences is no easy task.
Some great advice from author Nicholas Rossis that he passed along in a recent writers conference stated that the main elements to include are your main character, what s/he wants, what’s in the way, and the consequences of failure. Whatever you do, don’t include any spoilers or too many extraneous details. You want the reader’s curiosity to be aroused enough for them to want to read the story.