Lamkin's 1919 History of Henry County is available at the Missouri Digital Heritage website,
in the Missouri: Its History, Geology, Culture
Collection. Go to Browse and then scroll down the list of books, which is alphabetical by author.
It is now also available in online or in several dowloadable forms at
archive.org,
Sanborn Fire Maps for Calhoun, Clinton, Deepwater, Montrose and Windsor for
various dates from 1885 to 1918 are at UM's MIssouri Sanborn Fire Map Collection. Set the Search Option to 'Title' and search on the city name.
Google is in the process of scanning and digitzing an enormous number of books, including many of use to genealogists.
Most books printed before 1923 are available in full text.
archive.org also provides an extensive collection of scanned books in downloadable form;
many are duplicates of those scanned by Google, but they also have books not available elsewhere.