Check out the book trailer for Double Life (made it today (finally) in two hours). The effect is best in full-screen. Even if it takes some extra buffer time, it's worth it.
This is not your grandmother's book trailer.
I did this with so much attention to it looking good and being short and sweet because so many people don't do book trailers very well. When I see a book trailer that has Windows MovieMaker effects on top of their Comic Sans-font text that fills up the whole screen, it makes me want to rip my hair out. Add in some Google image search results and you've got the average, made-in-5-minutes promo.
I like to call it a demotion, since you're lowering your audience's expectations for the book. Look, I know what they all say--book trailers move books. But really, if you can't make it good, it's going to end up hurting more than helping.