.ipk is for a hombrew called imageviewer, but you shouldnt need to worry about that since moonshell can view all pictures without needing to convert them to .ipk. And ComicBookDS also views images, not txt, just Google it.
I believe there's a homebrew application that allows for ebooks to be read in a similar way of how the Brain Training reading exercises work. Using moonshell to read a .txt file is a bit of a pain.
Here's the link: http://giesler.biz/~bjoern/downloads/readm...adMoreAlpha.zip