According to a post on Destructoid, the bonus comes in the form of a Limited Edition character art book. There's an emphasis on the word "limited" here, so go pre-order the game if you haven't already.
For those unfamiliar with the series, the Fire Emblem games feature epic sword-and-sorcery stories with turn-based strategy RPG gameplay. Players take turns moving their characters around a grid and trading blows with the computer.
Characters can use swords, axes, lances, and a variety of similar weapons in a rock-paper-scissors hierarchical style pf battle (swords beat axes, axes beat lances, etc). These games can prove difficult for some players because character deaths are permanent, resulting in repeated restarts of a level in order to save a favorite party member.
The game will be out for the 3DS on February 4th, 2013, so the window for pre-orders isn't very big. Check out the trailer below and decide whether or not you'd want a part of the tactical action. If the answer's "yes", then pre-order to get secure an art book!