Trailers are about temptations. They can be for a place, a product or an idea. We see them all the time in political ads as they try to convince us how we should vote. They are a marketing tool created either by professionals or those who love their products/candidates/place.
What I didn't know until 2012 was that books had trailers. Of course, movies did, but who knew books did. Where I am not totally certain how much a trailer sells a book, I do know they are a love song from the writer to their book.
What a trailer is intended to do is help a potential reader understand what the book is about, the energy within its pages. It is creating a mini movie which might not be so much about the plot (but could be) but about the emotional issues the reader will find within.
After getting started with this medium, collecting music and images, I found I had to do one for every book, including four of mine where I have not decided to release as eBooks.
To do a trailer, you have to search music sites like JewelBeat or Purple-Planet where you either pay a small amount or get it free for passing on its link. I also use my own images from photos and then those I have bought through varying Royalty free sites-- where if you know you need a lot, subscriptions are the way to go.
I only wish more models with the potential would put their images out. as there is a tremendous market for covers and trailers to go with the eBooks.
Recently I set these up to more or less go in the sequence of the stories, not when I ePubbed the books.