Amazon Kindle lending lets you share e-books
To lend a book, go to Manage Your Kindle on the web and click the "Loan this book" button. From there, just enter the recipient's email address and name, and add an optional note. The recipient has seven days to open the email message and accept your loan. After that, the book will be returned to you. Just like a paper book, you can't read your copy while someone else is borrowing it. (Of course, you can lend a paper book more than once, to as many of your friends as you want.)
Note that some Kindle titles aren't available for lending, because the decision to enable it (or not) is up to the publisher of each book. If there's no "Loan this book" button next to one of your purchased books, it's not lending-enabled. As for books you haven't yet purchased, the Amazon.com page for each one will let you know whether it's lendable.