You might try a SD card. You can get a 2GB for $7.99 & a 1GB for $4.00
here.
I agree with Harry that it seems like a file corruption or possibly your Cybook storage is nearly maxed out. Is yours the 64MB or the 512MB & how full is it?
Anyway the SD cards are very cheap and putting your books on the SD may help. I'm using a 1GB SD card and have about 250 books on it. The 2GB that Cybook sent did not work with my Cybook but works fine in my camera.