http://www.streetswing.com has some good stuff on the history of various dances, plus some step charts and other stuff. Also http://www.dancescape.com must have some useful information, although I haven't looked into it myself.
Also, take a look at our website directory at https://www.ballroomdancers.com/Directory/Websites/content.asp. In particular, look under the category of "general information", which is a list of websites having just the kind of information you're looking for.