I know nothing about them, but that will change. *evil laugh* If you go onto YouTube or some such, you can find book trailers.
I'm wondering if this is a new trend. I've not seen them before but happened upon one by Cinda William Chimas for her latest book, 'The Gray Wolf Throne.' http://www.youtube.com/watch?v=bXksII_uVG8
Not sure who makes them, either.
Her's seem to be done by DisneyHyperion. There are a truckload of them at that site. http://www.youtube.com/user/DisneyHyperion?blend=8&ob=5 Especially the Percy Jackson books.
I get the impression that some authors make their own vids. Interesting.
I'll research this and bring it up with my other writers' groups. Hopefully, we'll get the scoop. I'll get back to you.
(As if I don't have enough to do what with the story, the map, the synopsis, the cover letter.....)
Life is complicated.