Genealogybooklinks.com is a good source of free digital books of interest to genealogists.
It contains links to more than 20,000 online history, records, biographies and family genealogy books gathered from more than 24 sources.
Books are organized by name, subject and state in an easy to use browse able format. Biographies and family genealogies are arranged alphabetically by name. Some states such as Mich, NY, and Mass, are more developed then others. Genealogy Book Links is also a good source of civil war regimental histories.