If you place your pre-order for the DCC RPG prior to April 1st, you get a copy of a free pdf version of the book and the first 1000 pre-orders also get a free copy of DCC 65.5 Doom of the Savage Kings, a 1st level dungeon crawl. This module will not be available for sale afterwards.
Additionally, Goodman Games also announced a the release of a limited edition of the DCC RPG with a gold foil cover. This book will be a single print run available only at the release of the game. Pre-ordering the gold foil cover also gets you the limited DCC 65.5 Doom of the Savage Kings adventure.