Sure, there’s no official eBook software available for the Nintendo DS, but you do have a few options if you need a break from Mario Kart. The DS does have WiFi and can browse the web for reading content. What about offline content? It won’t work for any material with DRM protection, but you can try ReadMore, an alpha (as in pre-beta, so be cautious) application that displays .txt files on your handheld gaming system. You can hold the DS sideways with ReadMore, making it similar to a traditional book experience: book pages are shown on both the left and right screen in this orientation.
More into graphic novels and comic books? Check out Comic Books DS, a project over at Moon Books that looks pretty slick and takes advantage of the nice color screens on the DS. The typical downside applies here: you’ll struggle to find comic book content for the reader. There is a nice app that will convert scanned comic book images into the right format, but scanning comic books could be a painful chore.