In my survey of book trailers this past summer, I discovered that I'm not terribly fond of what are sometimes know as "synopsis" book trailers. These are trailers that try to summarize a plot. They seem to go slowly to me, and I often don't stick around to the end when I'm watching them. Perhaps I have attention span issues.
I prefer what I, at least, call situation trailers. These trailers simply expose the basic situation the book deals with. They seem to move faster, maybe because they aren't trying to do as much.
Two examples of what I mean by situation trailers are those for Red Thread Sisters by Carol Antoinette Peacock and Pretty Crooked by Elisa Ludwig. These trailers seem to show us what these books are about and not what happens in them. I say "seem" because I haven't actually read either one. On the basis of their trailers, I definitely would pick them up to take a look if I came upon them somewhere. That is the next step to reading them or, better yet, purchasing them. So for this viewer, the trailers have done their job.