This biographical sketch is written as it appears in the reference book; however there is no Muskingum County, Pennsylvania. I believe they meant Muskingum County, Ohio, perhaps a Cooper researcher can clear this up.
From the book "HISTORY AND BIOGRAPHY, COUNTIES OF WHITE AND PULASKI INDIANA. Historical and Biographical. Illustrated. Chicago; F. A. Battey & CO., Publishers. 1883." page 360, Monon Township, White County, Indiana.
"WILLIAM COOPER was born in Muskingum County, Pennsylvania, January 15, 1828, and is a son of THOMAS and TACEY A. (SAMPSON) COOPER, also natives of Pennsylvania. When WILLIAM was about two years of age, his father moved to Champaign County, Ohio, where he purchased a farm, but where WILLIAM had but spare opportunities for schooling. He worked on the home farm until he was twenty years of age, when in March 1848, he was married to SARAH HESS, born December 25, 1827, daughter of PETER and PHEBE (COLLINS) HESS, which union was blessed with thirteen children - SAMUEL F., infant boy (deceased), MINERVA (deceased), MARY (deceased), ANNA M., ISAAC, LOUISA, LYDIA A., JAMES, CALVIN, WILLIAM C. H., JOHN S. (deceased), and MARTHA E. In 1859, MR. COOPER removed to Madison County, Ohio, where he farmed until 1866, and thence to this township, where he purchased 100 acres of wild land. At this time, by prudence and industry, he has added to the same until it numbers 220 acres of excellent land. He is said to raise the best corn in the county. He is a Democrat and a good citizen."