A book trailer is a helpful marketing tool to promote your book. Adapted from the term movie trailer, a book trailer is a short promotional video distributed across multiple platforms to help you increase your online presence and reach a wider audience. It also gives readers a unique way to be introduced to your book and marketing message, other than a traditional book summary or review. While a great book trailer can be a powerful addition to your book marketing campaign, a poorly-designed, unprofessional book trailer can do more harm than good.