It's so good have these discussions about driving, especially for those that are in areas without in-person resources! I highly recommend Andy Marcoux from Coachman's Delight.com You can go to his website and there are articles to read and also downloadable lessons for small fees. I made myself a binder to put all of my purchased lessons in as I get them or have a need for them, and it has been one of my greatest resources.
I had the hardest time understanding the concept of contact and when to do what, when I first started. I paid for the lesson "Connecting With Contact" which is a PDF that you print out and he is so good at explaining it. I made a rein board from his tutorial on You Tube and that gave me a feel and better understanding too. So my rein handling is still developing and it is something that changes throughout the drive, and of course from horse to horse. He also discusses what to do with the slow horse, but the whole thing is just really good!