Book trailers are like movie trailers, in a way. Movie trailers cut and piece from the movie to give you a taste of what you will see when you see the movie, to lure you into the theater to buy a ticket.
Book trailers are a video advertisement for a written book. They cut and piece from the book -- add sound, action, pictures, sometimes live actors -- and turn the result into what would be a mini-trailer for the movie the book would be if the book were a movie. The hope is that you will see the trailer and buy the book.
Came across this one just now and thought it was a good example.
There was a panel that included some talk on book trailers at LCC a few weeks ago. You can make your own trailer for something less than $100 (and up) and post it on Facebook or your blog or you can pay one of the companies that know what they're doing (Circle of Seven Productions was specifically named) and for something in the range of $2K-3K (although the price can be much higher, depending on your needs) you get a professional video and the placement/marketing expertise of the company. Or you can make your own trailer and contract with a company like Circle of Seven to do the promotion.
Book trailers as advertisement. As lures. With hopes that the trailer will go viral and the fever will translate into sales.
This trailer (Michael Connelly, THE BRASS VERDICT) is a more sophisticated production with screenplay and actors.
Would you buy a book from a book trailer?
Do you ever send them on?