What is a good book to learn music theory...i have been playing since 4th grade and i am now in 10th and i want to start learning theory so that i can become a better musician

Go to your local music store, the sales people in the store can help. After all they are musicians themselves (or they should be anyway).

i have a basic book that i like a lot, its called Harmony and Theory: A Comprehensive Source for all Musicians

I have just taken my Grade 5 theory exam. I used 'Music Theory in Practice (Grade 1-5)' by Eric Taylor and the 'The AB Guide to Music Theory' to accompany them.