When you buy a book using your NOOK, you're downloading an eBook, but that eBook is very different from a physical book on one important way: eBooks are protected with DRM (see the tip on page 81), while physical books aren't. DRM essentially links a book to your use on the NOOK Tablet, or any NOOK reader, such as one for a PC, smartphone, or tablet. So you can't simply lend any NOOK book to anyone, or borrow any NOOK book from anyone. The book has to be coded to let it be lent, and not all books have this coding embedded in them. Why is that coding necessary? Without it, any eBook could literally be given free of charge to millions of people, by someone simply posting it on the Internet, and other people downloading it. If all books were available for free like this, publishers couldn't stay in business, and authors couldn't make a living (includ- ing yours truly). Bottom line: There are certain restrictions and rules about which books can be lent, how they can be lent, and for how long. With that background out of the way, it's time to take a look at how to borrow and lend books, first from your local library, and then between friends.