I remember a great book about pronunciation that I used which skyrocketed my pronunciation to new levels, it was called "Get Rid of your Accent". Don't be fooled by its highly marketable title, they claim it's used by British actors who want to learn RP to interpret certain plays that require it. I don't know whether that is true or not, but I remember it being very thorough. It came with a CD and it covered all the vowel sounds, semi-vowels, consonants, diphthongs and triphthongs (fire, oasis), and so forth. I highly recommend it.
You can find it on Amazon and in other places of dubious legality...
Incidentally, this is not spam and I'm not a teacher.