Mastering is a better "how to"... Enter is the original and goes more into the "science" (you can decide how scientific it is) of the Zone.
So it depends on what you're looking for. In retrospect, I'd probably have gone with Mastering. I've read both, but neither understood all of the science, nor necessarily bought into all of it.
On the other hand, I've tried the Zone (and still at it) and it works; Mastering was plenty for that.