I haven't bothered using the free ebook reader for my Dingoo as I use a Palm M500 for night time reading and a sony reader during the day. I have however read in a couple of places that some people are having difficulty formatting their text files for the dingoo.
I use a bit of software called Interparse to format my text files.
Interparse is no longer supported but I found a link to it here. http://www.mobileread.com/forums/showthread.php?t=710
Simply load the file of your choice and click a few options. such as:
remove blank lines
insert blank line after paragraph
In the extended options is the rather useful convert extended ASCII and delete extended ASCII which gets rid of those annoying weird characters that files sometimes have.
Once you have the file looking ok save it with a .txt suffix and Bobs your uncle. A nice clean text file.
I've yet to find a file I couldn't clean up with interparse.
Did I mention it has multiple do and undo