If you're not familiar with the University of Missouri's Digital Library, it's high time you were introduced. There's lots of stuff there of interest to genealogists, including county histories, Civil War histories, plat and Sanborn maps, etc. From the official website:
University of Missouri Digital Library:
a repository of texts & images, hosted by the University of Missouri System.
What can I find here?
The materials in our repository are divided into text and image collections. Books and other printed materials which consist mostly of words can be found in the text collections; image collections consist primarily of images, such as post cards, newspaper images, and picture books. You can search them all from here, or search and browse each collection type individually.
What is this place?
Founded in 2001, the University of Missouri Digital Library provides a repository for digitized items on behalf of the UM Libraries. Over 20 text collections, and 23 image collections, have been added, including collections from 15 libraries around the state of Missouri. We provide negotiated free storage for digital objects and metadata, as well as a search interface, at no charge to any Missouri library, museum, or historical society. UM Library Systems keeps all this stuff running smoothly.
We maintain a list of other collections, including other University of Missouri Digital Collections, which we invite you to explore. If you are interesting in historic newspapers, you should check out our Missouri Historic Newspapers repository. We also have a handful of eBind and PDF files, created in the early days of the digital.library. We will incorporate these older collections into our text repository, but you may currently view them as-is.