I got a bookstyle cover for $30 at my local Coles. It seems good - all the right spots are open (for charging, sd card, power, volume), but one thing I noticed is that the stitching on the inside of the cover leaves marks on the screen. Can't really see them when you power it on, but it's annoying - like fingerprints. That's what I love about my Kobo WiFi...I can eat sloppy chicken wings while reading and my screen still says perfectly clean!
I don't think the stitching will do any damage to the screen, but now I am thinking a hard sided case or sleeve would have been better... ???