I currently have a board for several of my books. I like being able to explore what the characters, scenery, and specific objects in the book look like. I also like sharing quotes that relate to the content of the book when possible. For my "No Rest" board [http://www.pinterest.com/innesjen/no-rest/]I have several images for each character, I have a lot of images for the scenery in the book and I've shared lots of quotes that set the mood of the world these characters live in.
I'm also able to use Pinterest to drop hints about the story with seemingly random images such as the one below from my "No Rest" board:
Pinterest also gives you the opportunity to save images for books you haven't written yet but are in the back of your mind and you want to remember key things about. Or to potentially even explore the story through images prior to setting a specific plot. I have a few boards for books that I intend to write and I use the boards to keep myself excited about the project.
Pinterest is also a great way to engage with readers. By sharing your Pinterest boards with readers you can help them get excited about your books, the world they are set in and the characters that you've created. Below I've attached an image from my board for my book "Two Heads Are Better Than One" that represents a key object from the story. [http://www.pinterest.com/innesjen/two-heads-are-better-than-one/]
There is an added feature of creating hidden boards. These boards can be used for images you want to be able to see but you don't want others to see on your site. These boards can help you get excited about a project before sharing it with everyone else. They also can be used to only allow a select few people to see the images you've saved there.
Lastly, Pinterest also allows you to explore your brand online and to share things that associate with your online persona, or at least to share some of your other interests with the public at large. For me, that means lots of cat photos [http://www.pinterest.com/innesjen/gabby-and-chewie/]: