Originally Posted by angushay

I bought a book a while ago called Gradus Ad Parnassum, by J.Fux. This is arguably the most helpful book for the study of counterpoint. It's a few hundred years old, but has been studied in full depth by J.S.Bach, Mozart, Beethoven. This will make a huge difference to anyone interested in writing counterpoint.

Hope this helps you as much as it helped me!