I am fully capable of speaking, reading and writing (British-)English and my native language, but I find that I tend to mumble and muffle my voice. Furthermore, I would love to perfect my speech in all the ways possible, and I find that the science of articulation (articulatory phonetics) would be the best ways to do this.
I don't want to hit up random books on Amazon, so do you know which books are the bibles within this areas?
Maybe theatre-oriented books would deal with articulation and contain some exercises?