Amazon is clearly the 900 lb. gorilla. Barnes and Noble appears to be firmly locked in second place as an online book retailer. The problem for Barnes and Noble, the giant of brick and mortar chain book stores, is its second place ranking is not close to Amazon when it comes to sales volume.
I have spent a lot of time researching how to maximize sales for my books on Amazon and certainly follow the POD publishing model of trying to drive the bulk of my sales to Amazon to take advantage of the virtuous cycle and the long tail. I am still trying to figure out how to use the features for authors on Barnes and Noble to maximize the opportunities for selling books on Barnes and Nobel. There are no books such as Aaron Shepard's Aiming at Amazon to provide the details on how to maximize sales on Barnes and Nobel, probably because Barnes and Noble's market share is not large enough to justify writing a book on the topic.
Still, I do believe in the concept of providing your potential customers with as many different options to purchase your product as possible. In an effort to expand the options available to potential customers, I have expanded several of my titles distribution by signing them up for Createspace's Expanded Distribution Channel. My book on self-publishing using CreateSpace, Self-Publishing with Amazon's CreateSpace, is now available on Barnes and Noble. My share of the profits from sales on Barnes and Noble will be less than if the same title sold on Amazon, but the sale would be one that I would not have made otherwise.
While I am an advocate of using CreateSpace, the same option of selling books to online distributors is available when using Lightning Source and this approach has the advantage of your, the author, setting the discount you want to provide to the online retailer.