Many are familiar with the National Register of Historic Places. A good resource for research is the set of nomination forms for places on the Register. These are available from the National Park Service at the address on this page.
The Park Service hopes eventually to have all of the nomination forms online. But until then, a e-mail to the Park Service will get you the materials. They responded to my e-mail within a day or two and sent me materials in less than three weeks. I was not charged for the material.
So what does one get? Well, I sent for the nomination package for Buena Vista, the plantation in De Soto Parish, Louisiana, where my Brayboy, LeJay, and probably Gines, ancestors were held in bondage by the Boykin Witherspoon family. The nomination papers included a written statement of significance prepared by the nominator. This statement described the property and included an historical note that contained a transcription of the1859 contract to build the estate. There were also three pages of photographs and two pages of diagrams.
Information similar to this can be found in the nomination papers of any of the properties on the National Register. If your family had any connection to a National Register property, these papers may add to your understanding of the family.