County histories have been touted as a valuable resource for genealogists.  Even if your ancestor isn't in the index, they are worth looking at for the background factor.  I want to understand the what, how, where, when stuff so that I will have a better understanding of what my ancestors went through, the landscape upon which they lived their lives. 

This reminds me of a book review that I recently read (thanks to Gale Power Search).   I recently started receiving my alerts again after a lapse of a few years; my search parameters were "family history" and genealogy.  The name of the book is Section 27: A Century on a Family Farm by Mil Penner.  I haven't really read the book, yet, but from the review I can tell it's one of those kinds of books that makes you feel like you're there.

Another good reason to dig this deep into county history is that it will permit you to design a time-line on a level that will be very up close and personal to your ancestor's time-line.  Our lives are tied to the places we've lived, right?  The same holds true for family tree folks also. 

Finally, I can't help but think that after you've completed a project of gathering and looking into county-level histories, or even before you finish, you will start to see little clues here and there--other sources of information  you haven't thought of previously to check.