What do you guys think of the almighty mental game? I've focused on this aspect of tennis relatively early in my years of tennis, around 12. I read and followed the principles of "The Inner Game of Tennis", and it had become my bible. I also bought another book, actually an ebook, on the mind game to compare and contrast. I thought it was an excellent complement to TIGOT, since it focused on self-hypnosis, which TIGOT doesn't cover. There's a link to it at my blog here.
I believe the mind game is about 80% of the game. After watching a Little League baseball game today, it truly showed how losing or winning is defined by your mental strength.