If a million people download the demo across PC, Xbox 360, and PlayStation 3 before March 1, two books will be granted in the full game. The Far Cliffs of Kirkwall gives some extra money, and Lothering's Lament gives an XP boost. The site does note that it will be counting EA account log-ins towards the tally, so make sure you log in if you want to contribute to the effort.
Of course, you already have a variety of ways to get special swag in the game. Completing the demo gets you Hayder's Razor, playing the Legends flash game unlocks five items, and a code in new copies of Dead Space 2 gives you the Ser Isaac of Clarke armor. All of this is on top of the extra sword and shield for pre-orders, the Black Emporium area for all new copies, and the Exiled Prince mission with the Signature Edition. Whew.