This is first-hand experience. My sister finally gave in and decided to use a Pocket PC. She is going on vacation, so I managed to convince her of the benefits of ebooks and last night we sat down together so I could show her how MS Reader works. She is a beginner and already happy that she managed to remember the basic functions of her new gadget, so she refused to listen to anything beyond the MOST BASIC STEPS.
So: I would keep the real beginners in mind, and add a lot of screenshots to the documentation. Maybe a
First-Steps guide would be good. Once users are ready, they can click on
Advanced Features or something like that.
For that, I agree with everything that was said so far. I cannot get enough information, as long as it is well-organized and I don't lose the overview. For me, it is important to know
if and how the program and the books can be installed/copied to the
memory card,
which file extensions can be read with the reader and
where books can be downloaded/purchased (BTW, thank you Microsoft for leaving your German speaking customers out in the open without a single option to legally purchase ebooks for your format!

and for
leaving the old links to 2 ebookstores on your German MS Reader page one of which only sells Adobe books and the other one being a small publisher's website that does not sell any books at all - great way to piss off customers!).
That rant leads me to the next point: you got a reader you are selling to non-English speaking folks? Make sure there is
content for them, and include a guide on
converting your own books in the manual, please!!! I got so used to converting my own content that I don't think I could go with a reader that did not let me do that - for free.