The Sanborn maps were created in real time as the Chicago area was growing very quickly and it was not a mapping project that was planned or organized. Instead it was done on an “as needed” basis for the benefit of the clients of the Sanborn Map Company. As a result, there can be a lack of logical organization. But generally we can find the areas that you are interested in seeing.
The area in Cook County is more complicated because in part the City of Chicago has annexed areas that were previously outside the city limits and when that annexation occurred has some effect on the volume numbers and/or letters. Sanborn maps for the Cook County area have numbered volumes starting with 1 in the beginning until the present with volume 49. The maps were originally just in the central part of Chicago, but as the city grew, the number of volumes increased. Also, areas outside the city were in lettered volumes and with the passage of time and growth of the county the lettered volumes increase as well from A through C, and, eventually, A through K.