Most digital-format books are formatted with restrictions to prevent people from copying and sharing them in this way, to protect the author's rights and, frankly, their income.
Of course, before ereaders, you (generic) could have loaned your MIL a book. You had paid for a physical copy of the book which you then owned and you could do anything you pleased with it. The publisher and author got their royalties, and because you, as the book's owner, were only ever likely to lend it to one or two people, no one ever worried about the book being read by people who hadn't actually paid for a copy of their own. What's more, you were unlikely to charge for the loan of the book, and profit from the author's hard work. (Special royalty arrangements exist for public libraries.)
In this digital age, ebooks without restrictive 'locks' in their software can be shared with, literally, thousands. Not personally by you, perhaps, but you send it to ten friends, who each send it to ten more, who send it to ten more... And so on. It's the nature of the internet, the whole reason for its existence, is to share digital information easily and instantaneously. The author gets one payment for one copy and loses out on dozens of sales. When this is their source of income, it's a real issue for them. So the copyright locks are put in place to prevent the easy sharing of books, to protect the author's rights.
However, because people do like to share books and because word of mouth enthusiasm boosts sales, Amazon does operate a limited sharing scheme where you can lend ebooks for a short period. I don't know if B&N has anything similar you could explore.
That isn't to say the digital locks can't be broken. Pirate sites pop up every day that sell copies of ebooks, and the author will never see a penny in royalties. Heartbreaking, when your work is stolen and someone else is making money on it, and you're struggling to make the next mortgage payment.
The other thing to note is when you buy an ebook, you don't actually own it. I believe that what you are technically buying is a licence that allows you fair use of the electronic copy. I do know that Amazon (and propably B&N) can remotely wipe books from your ereader if you breach the terms and conditions on which you bought them.
Which boils down to 'there is no easy or legal way to share ebooks'.
You can load up with books that are out of copyright. Check out Project Gutenberg, which has 1000s of free books. Older books, of course, but there are some gems there.