There is one book I can recommend as far as composition is concerned. This one: http://www.amazon.com/Twentieth-Cen...sr=8-1&keywords=20th+century+harmony
And there's also this: http://www.amazon.com/Materials-Tec...sr=8-3&keywords=20th+century+harmony
which I haven't read, but appears to also be very good (and will order it once I finish this post from the US... It'll take a while to get here, but what the heck... It's Christmas, I deserve a present).
In my teachings as a composer I use the first book, extensively. Its break down of various subjects in harmony, cover a lot of ground!
I would think that getting a teacher would be the best approach, though. Composition is a very open subject and one can get lost without some guidance (without this meaning that the student should follow aesthetically his/her teacher).
Best of luck!