I get the bookreporter.com newsletter which always includes reviews of new books. To help you decide whether or not you want to read the full review, they summarize the book’s plot in two or three lines. I think these are good models if you find that you need to do this kind of summary—for instance, in a query letter to an agent or a publisher.
Here are two examples:
EYES WIDE OPEN by Andrew Gross - Jay Erlich's nephew has been found at the bottom of a cliff at Morrow Bay. It's all just a tragic suicide --- until secrets from the past begin to rear up again. Did a notorious killer, jailed for many decades, have his hand in this? With eyes wide open, Jay puts his life at risk to uncover the truth.
This description tells us quite a lot: the genre, the setting, and the mystery at the heart of the book.
KILLER MOVE by Michael Marshall (Thriller) Bill Moore already has a lot, but he wants more. Much more. One morning, he arrives at work to find a card waiting for him, with no indication who it's from or why it was sent. Its message is just one word: modified. From that moment on, Bill's life begins to change...
By way of contrast, this description doesn’t give us many specifics, but in a way that’s what makes it intriguing. My only quibble is with the last sentence: “Bill’s life begins to change…” is a bit weak. I don’t know the story, but something like “Bill’s life begins to come apart” or “Bill’s life turns into a nightmare” or “Bill is forced to question every part of his life” would have been stronger.
It can be useful to come up with this kind of pitch even before you write the book, as a foundation (answering the questions I mentioned in the post just before this one can help you do this).
(There are more tips on writing your book in "Your Writing Coach" published by Nicholas Brealeay and you can get it right now via Amazon or your other favorite online or offline bookseller.)