This topic has been done to death already.
For the purposes of this site it is not.
I'm not a fan of using any outside material when playing someone, not even for the opening.
Technically though, I'm not sure its considered cheating in correspondance chess.
I wish people would bother to learn the rules before starting threads like this.
maybe there should be a short mandantory quiz on the rules when you join, with easy interactive popups for the right answers so even a complete beginner could learn them right away. sure it would add a few moments to the registration, but it would also wipe off 20% of the newbie threads easily. and we'd have better quality chess talk for it.
What do you think. Is using a opening book reference cheating or isnt it?