The one I used when I first began to study in 7th grade was Latin for the New Millenium. I found it to be very organized and cumulative, introducing finite chunks of syntax and grammar rules in each chapter and not getting too long-winded and confusing in its explanations (which I've noticed can be the case in a lot of the older, more famous textbooks). You should be able to work through it pretty quickly - I think there are two volumes.