The Starting Block of Music Theory (download)
This book is great for you if you already play an instrument and want to learn basic music and notation. There are many possibilities, with just a little study time music becomes easier to understand and more interesting. It is also a lot more fun when you know a little more.
So what are you waiting for? Just start reading and remember to use your instrument if there is something in the book you would like to try and play. It is always an advantage if you keep your instrument close at hand.