The Public Library of Cincinnati and Hamilton County has a growing digital book collection. One particular collection caught my eye. This one is filed on their web site as "Old & Rare Books : 900 - History, Genealogy & Geography."
I found family histories, city directories and telephone books, local history, military regimental histories, pension abstracts, DAR lineage books, and more. If you are looking for material from Southern Ohio and Northern Kentucky, this is a good place to start. The nice thing about this collection is that you have the choice of reading on-line or downloading the book to your own computer in a PDF format. It's free to access and use.