Begin with a photo of your book that has a blank area of one solid color off to one side. Include in this empty space the title and your name as the author. As for the kind of type, pick a font that is bold and large but takes up no more than a third of the space.
Write a brief synopsis that includes intriguing details from the story. This should fit under the title and author information. Use 50 words or less. Do not give away too many facts about the plot or too much information about the main characters. The point is to create enough interest so people will want to buy and read your book.
Obtain reviews from readers and critics. Remember to include names and publications of all reviewers near the reviews (The most common format is directly underneath the review in smaller font). On the poster, print a positive word or short phrase that connects with your desired audience and promotes your book or yourself as an author. Simplicity says it best in this situation. By limiting the text to just a few words from a few reviews, you can save your own time while saving space on the poster as well.
Add a short list of businesses and online stores that will carry your book. Put this list under the cover photo or even on the bottom of it. Although this isn't mandatory as far as poster design, it will go a long way toward promoting the sales of your book. Those who are interested will know where to buy a copy.
Add a photo of yourself to the poster. Place it in a "bubble" or circular shape in a corner of the poster, near the book cover photo.