First, if you haven't read it yet, Jonathan Sarna's American Judaism: A History is a good survey of the history of American Judaism. Looking at the footnotes and bibliography can be a good launching point for further research and reading.

Regarding the Reform movement specifically, Michael A. Meyer's Response to Modernity: A History of the Reform Movement in Judaism is probably the best comprehensive history available, and it includes information on both German and American reform. Dana Evan Kaplan's Reform Judaism: An Introduction is also useful, though it's not strictly a history because it also includes consideration of theology and contemporary practices.

Regarding your interest in the history of Jewish women leaders you might try Pamela S. Nadell's Women Who Would Be Rabbis: A History of Women's Ordination, 1889–1985, which is a history of the struggle for women to be ordained in the Reform, Reconstructionist, and Conservative traditions. Nadell also recently published a book America's Jewish Women: A History from Colonial Times to Today, which is a readable look at women throughout American Jewish history.