County histories were often published because of an event, like the 100th anniversary of a town or because of a larger celebration like the United States Bicentennial. To find a county or local history in the area you are researching, check out the Family and Local History collection available through Ancestry.com (this is a paid subscription) or through the digitized books available through Heritage Quest, which may be accessed at some public libraries.
If you are unable to find a county history for you area of interest, check to see if the Family History Library has one in their microfilm collection that you may borrow and use at your local Family History Center (www.familysearch.org). To search on the county or city level in the Library catalogue, begin by searching for a state, and then click on the button that says "related places." This will give you a list of the counties on which the Library has information. Once you click on a county, you can once again choose "related places," and that will take you to a city level. From either the county or city level, look under the category history for possible books that might be of help to you.
Another place to find county histories is through a library, either in your town or in the town in which you are interested. To find a library in another locale, you may want to access online databases such as Libdex (www.libdex.com), Libweb (http://lists.webjunction.org/libweb), or Libcat (www.librarysites.info/). What I like about Libcat is that you can select a state, then select a library, and you are taken directly to their website. If these options don't work for you, then simply Google the name of your town or county of interest and the word "library," If you find a library in the locale you are researching that has regional history books, talk to your reference librarian about borrowing them through interlibrary loan.
There are some limitations when researching county history books. The everyday townspeople usually are not highlighted in these books. People often had to pay to be included in these books. They paid to have a biographical entry and maybe even a sketch of their likeness. So this is one of the times in your research that you aren't selecting a resource for it's mention of your ancestor but what it can tell you about the place your ancestor lived.
In the newer county history books, published in the last three decades, family histories were often solicited and written by family members about their ancestor. These can be invaluable, but remember, they don't cite sources, so you will still need to verify "facts" contained in these narratives.
One statement I have heard in presentations on using county history books is that because a person paid to have their information included, it is always a glowing, praise-worthy report and not an entirely factual reflection of that person. Like almost everything in life, this is not a hard and fast rule. In one county history about my Bell ancestor's county, Clarke County, Alabama, a narrative written about an allied family member is anything but complimentary:
"Robert B. Patterson, in 1840 elected sheriff, was for many years a prominent citizen. He was probably the most successful and skilful in electioneering of any man in the county. His address and manners were very captivating. He would win favor and votes even from those determined beforehand not to vote for him. It is said that he never became angry, never could be caused to manifest a hasty temper. He carried on a large plantation north of Grove Hill and raised, in the days of his prosperity, one hundred bales of cotton a year. He acquired, unfortunately some bad habits. He drank and gambled, lost his property, and his bondsmen paid for him large amounts. He died at last an imbecile, in the county poor-house, his intellect, once bright and commanding, all in ruins, a dreadful example of the evils of strong drink."
Depending on the publisher, older county history books sometimes have beautiful sketches of a town and prominent places, including family homes, within the town. Newer books will contain photographs. A county history might tell you about farming conditions, geological features and what services were available. A historical biography of the county or towns within that county will also be included. Sometimes these histories can be slanted, especially ones written in the 19th century. As with any resource you find, it always a good idea to back up what you find in a county history with other documents. Although a glowing or derogatory narrative of an ancestor will be hard to back up with other evidence, the circumstance of their life; living on a poor farm, or owning land are facts that you can substantiate with other resources.