Book joins David Krikorian in the Democratic primary. Krikorian ran for the seat as an independent in 2008, taking a higher percent than any other independent running for federal office.
"In these tough times, the people of Southwest Ohio deserve a Representative who will be independent, work with both parties to solve problems, and stand up for Ohio values in Washington," Book said in a statement. "In the State House I have been an independent voice, and unafraid to roll up my sleeves and get to work on the issues most important to Ohio families. That is exactly the kind of leadership I will bring to Washington."
Schmidt holds one of the most Republican districts targeted by Democrats, but she has struggled to hold it in recent election cycles after winning a closer-than-expected special election in 2005 over Iraq veteran Paul Hackett.
Democrat Victoria Wulsin has lost each of the last two races in the district, but Schmidt was held to 50 percent or below in both races.
Krikorian raised a modest $60,000 in the second quarter.