Incompetech is wonderful. I used some of his free music, too. Glad you found him. Did you check out Partners in Rhyme? Good for gunshots, explosions, etc., although I had to download Audacity (free) for blending the multi-track audio.
J. Gunnar Grey
I've put together a number of trailers, one of which is on my profile page. It's quite easy and not expensive - in some cases, it's free. For music, I'd check out iStockAudio or AudioJungle. They have free downloads on occasion, in addition to the ones you pay for. They're made for commercial use, and are royalty-free once you purchase or download.
You can also download a lot of interesting background sounds and sound samples for free. For example, try Freesound at http://www.freesound.org/index.php. There are a lot of other ones.
I often make slide show videos with musical accompaniment. There are a lot of places to get free, commercially available photos. Check out Veezle, which is a free photo search engine, at: http://www.veezzle.com/.
You can see more of my "home grown" book and story trailers at: http://www.viddler.com/explore/bostonmargy/
Trailers are part of marketing. They'll be as effective - or ineffective - as you are as a promoter. That being said, a good trailer can peak interest and maybe encourage people browsing online to visit your site.
Last year I created a video trailer for my suspense thriller, ABSOLUTION. View it ABSOLUTION-Booktrailer
I bought the rights to the music at neosounds.com They have an excellent selection, which is not expensive. I'm a musician and the right music was really important to me. As for the photos, some were my own. Others I got at Fotalia.com
Yes, I've found it very helpful as a marketing tool. However, it's important to make it short and punchy. People just won't watch something much over a minute. At least I won't. Figure out exactly what you want to convey. Don't tell the story of the book, just hit the highlights. Include any awards and be sure to include the address of your website ... or wherever else viewers can buy the book. Hope this helps.