Fux is a standard by which all should be measured.,
This review is from: The Study of Counterpoint (Paperback)
Whilst Johann Joseph Fux's Gradus ad Parnassum is definitely a must for those interested in musical theory, composition or analysis, I would recommend exploring more modern texts first as it requires quite substantial knowledge of modern music theory and the development of compositional technique over the last three centuries in order to get the most out of it.
Anyone intending to become an amateur, semi-professional or professional composer should have this on their bookshelf. Mozart, Bach and Beethoven can't be wrong.