You can actually find it online.

Go to:

http://www.sos.mo.gov/mdh/

Then:

Collections-

Books, Pamphlets, and Monographs

then:

Missouri County Histories

Then Polk County.

There is also a later History of the county I did not search.

this is the Missouri Digital Heritage site- and a OUTSTANDING source for County and Civil War history.

Hope this helps...and enjoy the journey of discovery you have embarked on.

Michael I.