If you’ve ever purchased a new book from a bookstore, a secondhand book, or a textbook for a course, you’ve most likely held a book that has an ISBN number. Sometimes college and university faculty will emphasize the need to buy a book for a course with a specific ISBN number to ensure that everyone has the same edition. You might even have entered an ISBN number if you’ve gone onto a publisher’s website with the intention of identifying or ordering a book. Anyone who has ever had to use an ISBN number already knows, most likely, that these numbers can have 10 digits or 13 digits. Even though many of us have identified a book by its ISBN number in some fashion, it’s rare to take a step back and to think about the purpose and history of the ISBN number. We want to tell you more about the history of the ISBN and why these numbers can be extremely helpful to book collectors.