How is an e-book different from a regular book?

Now, I don’t mean the obvious…that one reads it on a tablet, etc., but rather as far as copyright laws and such. I started reading a book on my Kindle today, where the author had written at the front of the book that if you purchased this book, please do not lend your device to a friend so that they can read the book without purchasing it. But don’t we lend “real” books to our friends? What is the difference? Is the above request legally enforceable, or just a request from the author?