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.
Both Charlotte and Emily have had biographies written about them. There are many about Charlotte, starting with Elizabeth Gaskell's book The Life of Charlotte Bronte, shortly after her death. And an important biography was written about Emily, too, in the early 20th century when Wuthering Heights was rediscovered. But Anne is most often overlooked, or if she is noticed, it is as part of a trio. No one, to my knowledge, has given Anne the honor of a biography all to herself, which is a shame, because I actually think she was the best writer of the three, and shows the most development in her craft between the first book and the second.
What about you? Is there anyone you are interested in, either in the past or the present, whom you would like to read a biography about?