Cabarrus and Mecklinburg are two of the counties not covered in the book. But also, you only mention that you found them in 1870, so I am wondering have you checked records prior to that? An individual listed as free prior to the 1870 census is one that would have emancipation records. Remember that after the Civil War, all of the former slaves would show up living in a location, while prior to that only the white owners would be. If you are not sure if they were in fact actually freed prior to the end of the Civil War, the next step would be to check the 1860 and 1850 census in the locale you are looking for.