You can choose any default bookstore on your settings page. Simply click the drop-down, select one and it will automatically save as your default bookstore.
This will be the first site you see linked beside a book when on desktop, so clicking the "GO" button beside a book will take you to your default bookstores website. If you select ebay as your default bookstore for example, then click "Go" beside It by Stephen King, it will take you to ebays listing of It by Stephen King, and allow you to purchase it.
This feature is even more important on mobile. Due to space limitations, there is no drop-down and instead each book link takes you to your default bookstore. We use geo-targeting and use Amazon as everyone's default store. So if you are an American vacationing in France, and attempt to purchase a book via our links on your iPhone, it would take you to Amazon France instead of Amazon.com. By setting Amazon.com as your default bookstore, it means no matter where you are in the world, the book links will take you straight there.