Whether it's cheating or not only depends on what the players agreed upon before. I don't use books cause I dont want to play a live game against someone one day and lose because I dont have help anymore.
I use books after the game is over to check what went wrong and why. when I am actually reading chess literature while playing Correspondence chess I am trying to avoid any topic that is directly related to a running game.
It would be polite to announce to your opponent if you're actually using a book. If you see it as a study game it is fine to me... but if you use books and then cheer over a won game cause you think you actually did beat that person yourself you are a fool (but not a cheater).
I've always believed it was cheating until I started to play against stronger players who knew openings like the back of their hand. Realising I was probably going to lose before the game was started I began using the book.
Prior to this I had no real interest in learning openings. Actually its one of those scenarios where I'd like to but there are so many variations that I just couldn't face it. I have too many other things to do and the whole thing was kind of redundant considering some of the people I play over the board. Online it's a different matter.
I have discarded the idea that it's cheating because it gives me a chance to study openings as I'm actually playing Chess which is more enjoyable than reading about some dynamic opening and thinking 'I'll try that next time somebody lets me.'