Its INCREDIBLY comprehensive. My music theory teacher gave it to me (!!) because she knew i was really interested in learning more about music theory. Started reading it and it literally goes from the simplest stuff in the beginning to some rather complicated ideas near the end.
You learn all the chords, all the intervals, some history, and a ton of knowledge about what went on in the minds of the crazy good composers like bach and beethoven and mozart. Seriously recommend it. pretty cheap too.
although if you dont know how to read notes on a grand staff, i suggest you learn that first.