Mango, where much of my knowledge came from is a lifetime of riding horses in different bits, seeing what worked and what didn't work and learning how to handle a particular bit to get a good response from it. Some of it was taught to me by more experienced horsemen and some I learned on my own from horses' reactions to how the bit was handled, both in my own horses and what I saw in observation of others riding their own horses.
Another good place to start will be in the Tack and Equipment section here. There are a couple of posts that are stickied at the top of that section, one about snaffles and english bits and another that I did about western and curb bits. That will help to give you a start anyway.
Also, don't be afraid to ask on here. If you see a bit and you want to know more about it's function and proper use, post a thread with a picture and there will be plenty of people willing to explain how it works and what its proper usage is.