Sony has abandoned the North American market and has successfully transitioned the majority of their customers to Kobo. Today, Sony has issued a new firmware update for their PRS line of ereaders that remove the Sony Reader Store. They also have eliminated the ability to sync bookmarks and book sharing options.
When customers install the new update they have no ability to purchase eBooks anymore. Sony has confirmed that they will issue a final update in the next two weeks that will add the Kobo bookstore as an avenue to continue to make the e-readers relevant.
The firmware updates should be available to install via WIFI and are only applicable to users living in North America. If the updates are not pushed out to you yet, you can simply download them from the links below. Residents of the United Kingdom, Europe and Australia are discouraged from manually installing it, as they still have the ability to purchase eBooks from Sony.