Publishers Weekly recently interviewed a number of book publishers and publicists to survey how they felt about the effectiveness of book trailers. You can check out the article here: http://www.publishersweekly.com/blog/400000640/post/1210039121.html.
Here is my short response to PW's article:
Most book trailers are not effective. They simply don't attract viral attention. Videos only work if they are passed on to others. Book trailers as they are now produced rarely create that viral sensation. But many other videos do achieve pass-along value. That's what book authors and publishers should be creating -- not book trailers but truly viral videos.
What I teach in my Ten Million Eyeballs online marketing seminars is how to create truly viral videos, not flat book trailers. Those are boring. Viral videos are interesting, funny, challenging, involving, and pass-alongable (my new word :)).
Check out http://www.tenmillioneyeballs.com.