A book trailer should have the same effect as a movie trailer. It should highlight some of the main events, never give away the ending, and give the readers a glimpse of what adventure may be in store for them should they open the pages of your book.
But be careful. This element can be a powerful tool or a hindrance if done wrong. You want to make sure that you don't mislead readers into thinking the book is about something it's not. You don't want to give away too much. And you don't want to use any copyrighted materials that you don't own the rights to.
Here are a few book trailers that I consider to be well done, powerful tools:
When We Wake by Karen Healey
Rules of Attraction by Simone Elkeles
And here is one that is shorter but still effective:
The Dark Divine by Bree Despain
What are your favorite book trailers? What makes these book trailers "fantastic"? Let's me know what you think and I'll use your opinions in my next post about how to make a book trailer.