In the good old days when all books were either hard cover or paperback there was no such thing as a book trailer.
You decided if you wanted to read a book based on the the brief description on the back of the paperback jacket or the inside cover of the hard cover jacket. Also known as the book blurb. While book blurbs are still alive and well and are most often the deciding factor in deciding if a book is for you, in this day and age there are many more options available to snag a reader's interest in your novel.
If you are looking at ebooks, many now offer the option to "sample" the book - you can read the first bit of the book, usually a chapter or two, for free before you buy. Since you were always able to do that with print books this is a good option for ebooks to have.
But the book marketing option I want your opinion on today is the book trailer. For those of you who are unaware a book trailer is somewhat similar to a movie trailer. It is a short video promoting your book. Often the blurb and other text describing the book, or a few lines from the book that denote it's subject matter are used interspersed with still photos and an overlay of music and/or a voice reading the text that is shown plays as well.
This is a fairly new concept, and I'm wondering. Do you think new books need a book trailer in this day and age in order to compete in the ever expanding book market?