With rare exception, the best I've been able to say about any book trailer is that it didn't actively impair my interest in reading the book it was advertising. Though I have seen several that did actively impair my interest.
But I don't agree with the premise that I've heard from some corners, that book trailers are just wrong; that since they are using one medium (audio-visual) to sell a completely different medium (text), they are intrinsically doomed to failure.
I think they are doomed because, at the heart of it, most of them are being made by people who don't understand the video medium. Many of them use static images-- either cover art or stock pictures*-- and have some Ken Burns effect and kinetic typography to give some sense of motion. If there's sound, it's public domain music or stock effects. Whatever text it uses is either blurbs from reviews, or synopsis boiled down to Twitter-length. That isn't filmic; it's attempting to make a dynamic experience out of reading the back cover. Furthermore, since the people making them don't understand filmic language, they don't pace it properly at all. Trailers take up to two a half minutes to give thirty seconds of information. Add to that the frequent use of stock images and sounds, and the net result is something painfully generic.
An overlong, uninteresting commercial telling me how your book is so unremarkable, it can be reduced to key words and generic pictures? Well, I'll run to the book store for that!
But then, there is that rare exception:
This trailer for Warren Ellis's "Gun Machine" is narrated by Wil Wheaton and illustrated by Ben Templesmith. Right off the bat, it's loaded with some professionalism. That helps a lot. What else do we have here? We've got a solid narration that comes straight from the text, and it's loaded with hooks. I don't know if this is the very beginning of the book or not, but it feels like it is, and it's got me intrigued.
So, key point number one: instead of telling us about the book, it uses what the book is.
Secondly, we have original imagery that is specifically created for this video. Even though the images themselves are static, it is made dynamic by seeing them being created. They tie directly to the text. It enhances what we're hearing. It's specifically engaging and relevant to the audio.
It clocks in at 2:30, and at no point does it feel like it's wasting my time.
That's how it should be done.
*- Which are often the same thing, sadly.