What I’ve learned is that a good book summary includes three things–it introduces the main characters, the setting, and the conflict. You don’t want an in-depth discussion of any of these things (after all, most casual book browsers will move on after a paragraph or two) but just enough to raise questions in your potential reader’s mind that he (or she) wants answered.
You can learn a lot by reading descriptions for popular books in your genre. If you keep an eye out for the three key components, you’ll nearly always find them.
As an example, here’s the description of my first book:
“All Anna Belko wants is a quiet cup of tea. (The opening line introduces the main character.) For a young garment factory worker in the uneasy city of Dor, such moments of peace are few and far between. (And this bit describes the setting.) When she stops by a little cafe called Oxsana’s on her way home from work, she has many things on her mind, but meeting the man who will turn her life upside down is not one of them.
After his cousin is killed, Boris Merkovich wonders if he will ever feel a moment of peace again. (The most important secondary character is also featured.) As the manager of Oxsana’s, his family’s cafe he doesn’t have time for grieving. But everything changes when he stumbles and almost pours a pot of tea into his customer’s lap. To his surprise, the young woman doesn’t become angry…
Love comes in unexpected ways. Neither Anna nor Boris dreamed this unexpected encounter would change both their lives, but as Dor implodes, Boris and Anna’s relationship, begun over spilled tea and a heartfelt prayer, only grows stronger.
However, it isn’t long before Anna realizes the hatred destroying her city isn’t just “out there.” If Boris cannot forgive his cousin’s killer, it will cost both him and Anna everything they’ve begun to hope for. (The blurb concludes with a hint as to what the main conflict will be, and hopefully raises questions about what will happen.)”
This was the first blurb for the first book I ever wrote, but still I think it’s fairly good. The reason for that is, I had a lot of help. Treat your book blurb just like you’d treat any other part of your story, and make sure you get plenty of advice from people who know what they’re talking about before sending it out into the world.