I thought this was a helpful article. My Ivan will occasionally lean on the bit, something he was trained to do during his racing days. We are currently re-training him not to do this with fairly acceptable success, but he occasionally forgets. Reading through this article was helpful and allowed me to rethink his bit and make sure it's still the right one for him.
The Horse Bits: Pain in the Mouth
This was a video recommended by SmartPak. Though focused on western bit fitting, it does have principles that are applicable to both western and english bit fitting and is helpful :)
The Horse Western Bit Fitting (video)
SmartPak's blog that lists a few different bits and also links to purchase them :) The write-ups on the bits are short but informative.
Smark Pak's Blog Entry: Bits + Pieces