Cubase Power. I can't tell you how much I learned from it. Even though I was already pretty well versed in Cubase. Cubase power is MUCH more indepth and complete then the manual. In fact, I find it easier to look things up and get answers in the book instead of the manual
To start out I bought the Fast Guide to Cubase SX by Simon Millward on PC Publishing. Got me started but I barely use it now as once the ball was rolling I quite like exploring and learning for myself.