I just stumbled across a Toyota Towing Guide. It is pretty good, and contains a lot of information on very diverse topics, including
What Is a WDH,
How to Back Up A Trailer,
State-By-State Towing Laws,
Electrical Connections,
Brakes, and a wealth of other good stuff. For some reason, Toyota hides it pretty well - it doesn't come up under the site's Search tool, for example. Of course it is Toyota-oriented, but it has a lot of stuff that is of general interest. I've always recommended Ford's Towing Guide (easier to find!), but in terms of overall coverage, this one seems to be even better.
It comes in three parts.
http://www.toyota.com/pdfs/towguide_part1.pdf
http://www.toyota.com/pdfs/towguide_part2.pdf
http://www.toyota.com/pdfs/towguide_part3.pdf
Bill