My first thought was that it could be tacky, as I've seen some tacky trailers before. So no, I didn't like the trailers I've seen, and I didn't buy the book they advertised (well, the reason also is that I didn't like the book's premise.
)
However, in truth, I think it can be a very effective promotional tool, but like everything else (book covers, websites, etc.), it should be done right. I do think the trailers should have a different focus than advertising for other visual media: TV, movies, video games, music videos, etc. while following the similar visual concepts. Books are more cerebral -- so we can't market it like a video game -- and you also won't have actual materials (such as screen shots of a movie or video game) to use as part of the trailer.
The problems I've seen with the other trailers are that they're too abstract and textual. Just because you're selling a book doesn't mean it should be textual slide show. "Show vs. tell" still applies here, but with books it's more difficult since, like I said, you don't usually have the actual visual materials to work with. But IMO, if you're going to cross medium -- doing a
trailer for a
book -- you may as well go all out with it. Get some stock footage. Shoot some video on your own. Get a soundtrack, etc. etc. Storyboard the thing.
That's what I'm doing -- I'm building a soundtrack for my novel and collecting stock video footage. Not that I'm going to make a trailer... I have no idea where I'm going with this WIP at this point, but at least it gives me creative ideas....