It's interesting that as soon as I say I'm about to get into writing articles about marketing, my friends at Indies Unlimited post November Video Trailer View-fest on their website. I believe that book trailers are a great marketing aid for your books. Beverly Cialone does all of mine, and she does a great job.
The key to using book trailers, I believe, is to use cool music, crisp pictures that fit your book, and to tease and not explain everything about your book. Trailers can be used as enticements when promoting your books in groups and on your own website pages. They can also be added to an Amazon link which gives a book description for added potential reader appeal. I like to switch around between all the above options as well as not saturating any one place with your trailers. There can be too much of a good thing. Also remember that spamming is bad. Be low key. Promote the trailers of other authors. Give them their due.
Another tip today is to use great websites such as Indies Unlimited. They run the gamut about everything an Indie author should know. I have spread around many of their posts and plan on doing many more. They are great people and enjoy helping the Indie community.
So, if you're thinking about doing book trailers for your books, I believe you are making the right decision. Check out the link above and see what other authors are doing.
By the way: I practice what I preach. My book trailer for Mists Of Papoose Pond is among the other great trailers on Indies Unlimited.