The e-Reader was a card reading device for the Game Boy Advance that would allow you to add extra content to specific games with cards that had special digital codes.
It was also complete bullsh*t.
You see, Super Mario Advance 4: Super Mario Bros. 3 here is an awesome game on its own. With the power of the e-Reader though, you could add a ton to the game. Enemies, items and power-ups from other Mario games could be injected into the experience that you never dreamed possible. There were even original exclusive extras such as a boomerang weapon that Mario could use. A boomerang!
So why am I calling it bullsh*t? Because only 1%* of Game Boy Advance owners bothered owning a freaking e-Reader, thus no one really got to experience these bonuses. Even less own any of the cards that were designed for Advance 4. Seriously, Nintendo, what the hell.
Also, the feather cape from Super Mario World is the best Mario power-up ever. Discuss.
*This number is grossly exaggerated by my nerd rage against the stupid stupid stupid peripheral.