American National Biography Online (Oxford University Press)
This is the standard biographical dictionary for American history. Bibliographies for each biography include memoirs, published letters, and archival holdings if available.
Take advantage of the "Research Ideas" option at the top of the search screen. One of the themes on the Research Ideas page is Civil War. Here you will find lists with links to biographical entries of important political figures, military leaders, financiers, industrialists and "recorders of and witnesses to the war." The last list includes a link to the entry on Louisa May Alcott.
Alternatively you can also do a simple keyword search for any person of interest or limit your search by "occupations and realms of renown." One of the options available is Civil War. Choose any of the suggested subject areas from the drop down menu.

African American Studies Center (Oxford University Press)
Includes the Encyclopedia of African American History: 1619-1895, Black Women in America, and the African American National Biography.

Dictionary of American History
Available online. The standard dictionary on American history.

Encyclopedia of the American Civil War
5 volumes, available in print only. The last volume includes numerous source documents such as proclamations by Lincoln and letters from soldiers.

Atlas of the Civil War
Available online.

Civil War: Primary Documents on Events from 1860 to 1865
Available online.

Historical Dictionary of the Civil War
Available online.