I can't really find much literature on using a GBA as an ebook reader. Can anyone help me out with this?

You have a Kindle. That will blow away anything you might be able to do with a GBA as a reader.

I'm just wondering.

You have to get a special cartridge to to this. I don't have it but I do have ebook reading capbility on my PSP and it is fairly rudimentary, a pain in the butt, and not at all worth the effort. I can imagine that the GBA would be worse.

If you are still interested look on ebay for a GBA video playing cartridge. I think most of these also function as MP3 players, ebook readers, and maybe even some rudimentary emulation.