The user manual of my Bookeen Cybook Gen 3 is DRM protected ! irrk..
Download last version of DRM Tools Archive here :
http://stream-recorder.com/forum/download-drm-tools-archive-t7955p3.html
Extract the archive, then :
tools_v4.7/Mobi_Additional_Tools$ python MobiDeDRM_032.py /tmp/XXXXX.prc /tmp/decrypted.prc
You can now open the file with fbreader.
I’m not sure it works with all PRC files. Maybo you have to give the device PID for some.