Sisters is the first major history of the pivotal role played by nuns in the building of American society. Nuns were the first feminists, argues Fialka. They became the nation's first cadre of independent, professional women. Some nursed, some taught, and many created and managed new charitable organizations, including large hospitals and colleges.

In the 1800s nuns moved west with the frontier, often starting the first hospitals and schools in immigrant communities. They provided aid and service in the Chicago fire, cared for orphans and prostitutes in the California Gold Rush and brought professional nursing skills to field hospitals run by both armies in the Civil War. Their work was often done in the face of intimidation from such groups as the Know Nothings and the Ku Klux Klan.

In the 1900s they built the nation's largest private school and hospital systems and brought the Catholic Church into the civil rights movement. As their numbers began to decline in the 1970s, many sisters were forced to take professional jobs as lawyers, probation workers, managers and hospital executives because their salaries were needed to support older nuns, many of whom lacked a pension system. Currently there are about 75,000 sisters in America, down from 204,000 in 1968. Their median age is sixty-nine.

In Sisters, Fialka reveals the strength of the spiritual capital and the unprecedented reach of the caring institutions that religious women created in America.