Most often we don’t think of the index to a book as a research tool in itself. Yet it is. It all depends on how thorough an index is and what exactly a so-called thorough index is indexing.
Have you ever looked at a 500 page book only to have a gut feeling only a fraction of the names in the book are in the index? I do all the time. One test I use to check the accuracy and thoroughness of an index is to pick some random names out of a book, and then see if they are actually mentioned in the indexed. Perhaps this is a little simplistic, but it works!
How you view an index depends on what the book is about and of course the geographic locality it is covering. For example, a top-notch index to some county deed books in Virginia will not only have the names of the: 1.) buyers and sellers, 2.) a section for slaves; 3.) a section for geography and watercourses; and 4.) include the neighbors mentioned in a transaction. Now this is a good index. Virginia does not have the township, section and range system, so we often rely on a mountain or watercourse and neighbors to sort through families with the same last name.
Now contrast this with the various land systems where you have township, section, range, or lot and concession or some other exact measurement. These are more straightforward. The need to list a watercourse is less since you can go straight to a map and plot the exact parcel of ground. So the research needs in the index would be different than in Virginia.
Going back to my Virginia example, I often use the geographical features and watercourse index to learn more about the neighborhood the ancestors lived. Remember, the earlier you go back in history, especially on the frontier, the more need there was for people to stick together. They immigrated together and then they settled among each other. Using this strategy, you may actually identify an entire community from the same place in Ireland. To find out where a few of them were from in Ireland would be to identify where your ancestor came from. Often this strategy works. Sometimes it doesn’t. However, you still need to try.
In using the surname portion of the index; take note of people with the same surname as your ancestors. It works a little better with uncommon surnames, but you can still work with the common ones like Smith and Jones. With the common ones, you would then take a map and sort them by watercourse or geographical feature. Those Smith families from opposite ends of the county are less likely to be related than are those from the same or nearby watercourse.
Don’t hesitate to utilize a well-developed index as part of your research strategy. I do this all the time, especially with published land and tax records. I get more positive results than I do negative. Your imagination is the limit!