The first book I would recommend reading would be "Musical composition" by Reginald Smith Brindle.
It is filled with great explanations on most aspects of composition. A great all-round book. I had a lot of use of this
when I was trying to develop on my own before I finally got admitted to a composition course (I still read it from time to time).
If you want to write contemporary art-music (I wish there was a better name for it) then I would recommend this book:
"Techniques of the Contemporary Composer" by David Cope. It really opens up your mind and gives tons of new ideas. I think that
this book could be useful if you want to compose film music as well, since you get more tools to work with.
And if you want to develop your harmonic language into something more modern then a book called:
"Twentieth-Century Harmony" by Vincent Persichetti
It can take a while to understand this book and you really need to play through his examples on a piano (unless your sight-reading is up for it, that is) to get what he is talking about. But it's really worth the trouble if you ask me. This book really made me realise how free you are to do whatever you like as a composer.
I would also recommend a orchestration book to learn more about the instruments and how to write for them. The one by Samuel Adler is my favourite, it's called Study of Orchestration. Unfortunately it's a bit expensive (and you should really get the discs with the audio examples for it).
on a side note, it's just so sad, that main three emotions (for music, and life?) are anger, sadness and happiness - basically it means that 2/3 of the music is really a dark thing. and you can get anger from sadness, and well, HTW happiness can come through all this?! this makes me sad and i'm going to write some angry music.
For me, I consider the melody to be the egg and the harmony to be the chicken. You can't have harmony without something for it to support, after all. I guess it's just a matter of perspective really.
I would say that the kind of rote memorization necessary for AP tests is counterintuitive to composition. Or actually learning anything. Me and my 4/3 on AB/BC calc did not equate to a C in calc II in college. Not to say that 4/3 is a great score or anything.