Hundreds of Iowa County histories - of varying quality - were written during the 19th and early 20th century. Now no longer in copyright (pre-1927), most of these history books can, if found, be accessed freely and publicly online.
Iowa State University Library Digital Collections has archived thirty such books in their Iowa County Histories collection.
To locate many of the other Iowa County histories not found in this collection simply perform a basic search (which defaults to “full view only”) in HathiTrust that includes the names of the county, Iowa and “history”
For example, to find an O’Brien County history search for: O’Brien County Iowa history
To find more recently published Iowa county histories - books only available in print - choose the Advanced catalog search option in HathiTrust
Set all the dropdown menus in the Advanced catalog search to “Subject” and type in the county name, Iowa, and “history” in the first three blanks e.g., Adams County, Iowa, and history.…
For books found using this method that are not available for free full viewing - type the title of the book in a WorldCat search e.g., “A Brief history of Corning and Adams County, Iowa”
Check the resulting record in the “Find a copy in the library” to see if a nearby library owns a print copy.
You can request print books owned by other libraries via Interlibrary Loan.