Hi Ivoch,
Congratulations, the Cybook´s a good choice. I really love mine
When converting plain text I take the .txt-file, use an editor and save it as html. I open the htm-file in Mobipocket Creator, press the "build"-button and will have a prc-file I can read on the Cybook.
If you want a "real" ebook (nicely formatted, table of contents, links etc.) from plain text or html, you can use Book Designer and Mobipocket Creator. Just follow HarryT´s tutorials on book creation which you can find here:
https://www.mobileread.com/forums/showthread.php?t=11678
and
https://www.mobileread.com/forums/showthread.php?t=17914