I have a small magazine sized "book". It has three levels plus the challange at the back. There are also a few that are called "extreme" where you have to have the "cross" rows with 1-9. My opinion, those are easier.
Since the game is popular there are a bunch of books out now. If you can find them I would recommend at least a easy, medium, hard options. Your brain needs a break now and then. Acutally, it kinda reminds me of cathe workouts!!!!
Jeanette