What is an ISBN?Every book on the market possesses its own unique ISBN (International Standard Book Number), a numeric identifier used around the world to identify books. If you plan to sell your book in a bookstore, through an online retailer, or to your local library, you’ll need to have a 13-digit ISBN number assigned to it. The process of getting an ISBN is not complicated, but there are important details you need to be aware of as you navigate the process.