See this thread for PDFs. I haven’t tried it yet myself, but it sounds like there’s a solution. 
EDIT:
I just tried this yesterday, and it works quite well! The key mappings from the setup script are also useful. With the 320x240 display, reading anything substantial will be a chore with lots of scrolling, but loading, zooming, and scrolling are nice and fast. That PDF viewer apparently supports ePUB files too, but I haven’t tried it and noticed it wasn’t included in the file associations in the script. Not sure I’d want to read an ebook on the GameShell anyway, but for quick reference or more visual PDFs without lots of small text, it could be handy.