Cook is a common US surname, according to the About.com article, "100 Most Common U.S. Surnames."
See WikiTree profile Catherine (Barnard) Cook/Barnard-1086. There aren't any sources posted to the profile yet. This person wasn't born 200 years ago and the profile is not othewise open. Barnard-1086 may only be edited by those on the trusted list.
The 1857 marriage of a George Cook and Catherine A. Barnard appears at least twice in the indexed-only Family Search Historical Records collection, "Iowa Marriages, 1809-1992." Click HERE. One record reports the marriage at Linn [County], Iowa; the other at Brown, Linn [County], Iowa.
The Linn location entry cites "Linn, Iowa, reference 2:2CBF61V; [Family History Library] microfilm 984046"; the other cites "Brown, Linn, Iowa, reference 2:3JNJ42K; [Family History Library] microfilm 1686055."
Both of the film numbers, above, are described HERE. Entries from film 984046 are reportedly from Linn County marriages, volumes 1-2 (1840-1869); those from film 1686055 are what appear to be alphabetized "Brides' marriage returns." (There is no doubt another indexed entry from the alphabetized "Groom's marriage returns" for George.)
Hoping only to be helpful, Brown is a township in Linn County--one of the 99 counties in present day Iowa. The place "South English, Iowa" (see Barnard-1086) is a city in Keokuk County, Iowa. Both Linn and Keokuk counties are located in the eastern part of the state.
The Iowa GenWeb project is is located here: http://iagenweb.org/ An especially helpful part of this site is their "IAGenWeb Quick Reference to: County Vital Records." Links to the county web pages appear on "Iowa County Formation Table."
Hope this helps.--GeneJ
See also what seems related G2G of 24 October 2013, "Who are the ancestors of Catherine Amelia Barnard (Cook )?"