All states who supplied men for the Civil War should have web pages such as the Iowa Civil War Project. It is a special project of the IAGenWeb and worth checking if you have an Iowa Civil War ancestor.
The web site contains the six volumes of Civil War Records for Iowa, the full citation being Iowa, Adjutant General Office. Roster and Record of Iowa Soldiers in the War of the Rebellion, Together with Historical Sketches of Volunteer Organization 1861-1866 Des Moines: E.H. English, State Printer, E.D. Chassell, State Binder, 1908-1911.
There is a list of correspondence, dates, sources and notes. You can explore this further for information pertaining to your ancestor or his unit. The web page contains Letters to Headquarters, such as Rock Island Barracks May 1864.
If you know your ancestor's infantry, be sure to check out the Infantry, History and Rosters. There is a separate section for Iowa Cavalry, History and Rosters, as well as Artillery.
The entire web page seems rather complex and time consuming, but help is available at the click of the mouse. Toward the top and before Correspondence/Topic/Title you will see "If you want to search all of these records go to the main Civil War page and utilize the FreeFind search engine there."
Not only can you search the site at the main page, but you can also find instructions for joining the Iowa Civil War mailing list. There are links to gravestone photos, more about the regiments, Civil War prisons, photo album, county data and descendant/researcher lists.
Plan on spending time at this web page, particularly if you have more than one Iowa Civil War soldier in your lineage.