just following M.H.'s instructions i fixed this re-booting problem i had today
due to my foolish deletion of some fonts ,i guess system fonts
....respect to M.H.
the steps are the following:
1. put out sd card
2. copy this file to your sd card
or to a new sd card in case it is corrupted
http://rapidshare.com/files/17053121...nelb538fmt.rar
3. put sd in your cybook
4. press reset button on the back side of cybook
5. press on-off button on the top of cybook
6. when the first screen appears keep pressing the central button (the ok button)of the reader ,until it starts updating the firmware
this formats the disk,but you need to.....
7. .....re-flash it with latest version from bookeen site :
http://www.bookeen.com/support 
and ..... buy your self a beer ...you have fixed it......it works again
and a last step if you are left with no Fonts folder and only two fonts
choice do the following :
Adding New Fonts
Problem: You've added a new font and now when you try to look at font families, it's blank or your new font isn't there.
How to make new fonts show up:
1. Plug the cybook into your computer and open the Fonts folder. If your unit does not have a Fonts folder in its internal memory then create one but make sure it has a capital "F" since it is case sensitive.
2. Add your new fonts. The Cybook officially supports TTF (True Type Fonts), but it seems that some OTF (OpenType Fonts) will also work.
3. This step is unnecessary when using firmware build 708. Go to the Cybook's system folder and delete this file: .fonts.cache-1. It will be automatically recreated. You can save a backup copy on your hard drive first if it will make you feel better.
4. Close all windows using the Cybook files and unplug the Cybook from the computer after using "Safely Remove Hardware" (or equivalent).
5. You should now see your new fonts available in the Font Family menu.