Book trailers - good idea or bad?
I think book trailers are probably a bad idea. At best, they're probably an ineffective marketing tool.
Your ultimate goal in marketing your book is to get people to read your sample. If they like it, hopefully they'll buy the whole book. So all your marketing efforts are basically intended to make potential readers aware of your book and entice them to read the sample. Once they find your book's page on Amazon (or wherever), the main tools to drive them to read your sample are your cover and your blurb. You need a decent cover and a decent blurb. If you don't have those two things, the potential reader probably won't download your sample and you've lost the sale.
I think book trailers are similar to the blurb and the cover. That is, a book trailer is a third way to jeopardize a sale. If they don't like your blurb, the odds of sampling decrease, and if they don't like the cover, the odds further decrease. Do you really want to add a third method to decrease the odds of sampling? If you're doing everything yourself and you're unable to make a decent cover or a decent blurb, odds are your book trailer won't be decent either. Or you may have a decent cover and blurb, and then your sucky trailer blows the whole thing. Then again, having a decent trailer alongside a bad cover and blurb may save the sale. But why take chances by adding a trailer to your marketing arsenal?
Readers want to read. Trailers are another medium entirely. Why mingle the two?
Let's say you do make a trailer. Isn't it hard enough marketing your book? If you're successfully able to make a potential reader find your book, you've already won the battle. Why waste time trying to make them aware of your trailer? Why not bypass the trailer and send them directly to the book?
I'm thinking along these lines: if you're talking to a potential reader, whether in a forum, on twitter, facebook or wherever, you've already got their attention. Why send them to a book trailer? Send them directly to your blurb or your sample. As a reader, I want to read, I don't want to watch a video. You may make a good trailer, but that doesn't mean your writing doesn't suck. In my quest for something to read, I'm looking for a good writer and a good book, not a good filmmaker. You're trying to sell the written word, not the visual picture.
Suppose you're a great writer but a terrible filmmaker, and you send me to your book trailer, which sucks donkey balls. You've turned me off and I probably won't sample your book.
If you're thinking of using a trailer on youtube or some other video-oriented site, then you've got the same problem you've got in trying to market your actual book: you've got to market your trailer. Because you can't just throw something up on youtube and expect it to go viral. Your video won't get many hits at all unless you make some sort of marketing effort. You then have a choice: do you want to spend time marketing your video, or would you rather spend time marketing your actual book?
Having a good trailer would be an asset, I suppose. But it's risky as to whether you'll actually make a good trailer. I think your time is best spent on making your cover and your blurb the best they can possibly be, because those are your two main tools. Adding a third is a big, unnecessary gamble.