Bust out your reading glasses: Halo: Cryptum, a novel by sci-fi author Greg Bear, is now available.
The first of what is planned to be a trilogy, Cryptum will explore the details of the devastating Forerunner-Flood War. If you don't know what that means, then this novel probably isn't for you.
But if it is, you can pick it up right now from Amazon in paperback, hardcover, and digital flavors. You can get a taste of the book for free online, too: publisher Tor Books has the first chapter online, and you can continue with the second chapter on Halo Waypoint.