See
http://booxusers.com/thread-278.html. The second link in the first post gives you the Bebook firmware. The link there still works, but you need to log in to the forum.
The most stable version is an Onyx Boox version, but the firmware from BeBook was also created by Onyx, and the two are exactly the same except for the logo. You can download it here:
http://www.onyx-boox.ru/files/OnyxBOOX60_20110418.zip