My ebook reader has become my best friend, partly because I've discovered how many classics and older books I can get for free (as well as the occasional free book for a publisher's promotion). Project Gutenberg is one of my best resources for free ebooks, and as I've discovered, there are many older biographies available on the website.
One that I think of readily is Anne Bronte. Most people who know much about literature know about the Bronte sisters, but how many people know that there were three of them, and that all three were writers? Charlotte Bronte's Jane Eyre and Emily Bronte's Wuthering Heights are the best known of the sisters' books, but their youngest sister, Anne, also wrote two books of her own: Agnes Grey, a short novel about a governess (based on Anne's own experiences), and her masterpiece, The Tenant of Wildfell Hall, about a woman who defies Victorian society by taking custody of her young son and leaving her alcoholic, unfaithful husband.
For instance, the book discussed in great detail Martha Washington's involvement during the war and afterward. It was her money, essentially, that financed many of the great things George did in the name of American independence. She was also a significant presence on the front during the Revolutionary War, and she and the other wives helped to keep spirits when things looked very, very bad.