Quote from TheJERTeam
Although I can not confirm this, it should work but you may find Essentials and CommandBook "fighting" over the same commands. You can choose which plugin handles which commands by editing bukkit.yml. I am unsure why you would want to run two command systems at the same time anyway? They both offer a similar service.
I couldn't switch over from Commandbook to Essentials..
How do I do that?