County histories were the 19th century's equivalent of today's "Who's Who in American Used Car Dealers"; if you bought a copy you were included in the book. Half of the "Biography" for a given individual was fluff. ("Travel where you will, you will not find a more hospitable greeting that the one you recieve at the home of Mr. and Mrs. Elliot McCorkle . . .") The rest is probably true, although people tended to touch up the truth a bit and believe the family legend of Great Uncle Elmer giving up his bed for George Washington.
The first place to check to see if the county you are interested in has a history and it is on-line is the US Gen Web site for the county. They start here: http://www.usgenweb.net/statelinks-table.htmlhttp://www.usgenweb.net/statelinks-table.html
Each state page has a map or table of counties, with links, which takes you to the county site.
The county sites vary with the skill, time and energy of the coordinator and his/her helpers. Just about half are above average, oddly enough.