Imagine yourself walking into Barnes & Noble or any other major book retailer. Perhaps you visit the store about once a month to browse a few books and grab a cup of coffee. You open the store doors and see in front of you one of the staff members sitting behind a counter. The person calls you out by name, asks how you are doing, and then proceeds to itemize the last five books you browsed when you last visited the month before. Not only that, the staff member then proceeds to tell you about several books of which you were completely unaware.