I recently reviewed a Mobile USB Power Pack that is made by Pocket PC Techs:
Pocket PC Tech's Lil' Sync USB Mobile Power Pack
The focus of my review was on how well it works with Pocket PCs, but, since it uses a mini-USB cable, on a whim I plugged it into my Cybook and found that it works with the Cybook too!
When I got my Cybook, I did not get the AC adapter, so the only way I've been able to recharge the battery is by plugging it into a USB port on my computer, and that's not always very convenient. Now I can charge up this little USB Power Pack and take it with me when I go on the road, and I don't have to worry so much about getting to a computer to recharge the Cybook.
Check it out and see what you think!