What would make you interested in a book? Reviews of it? Quotes from it? What would you like to see in a book trailer?
The most interesting part of a book is the climax. For the book trailer, it is best to get catching quotes from the climax and make it long enough so that the reader can somehow make a connection with the story.
definately the title and the cover those are what make me question hmmm this book looks good and once the cover catches my eye i read the back of it then i read it :)