books about the war, both secondary sources, histories of the war and so on, and many primary sources, for example memoirs of participants in the war. There are some major resources online too, like Cornell's digital edition of the Official Records of the War of the Rebellion. This site is the digital version of the print set, which records thousands of memos, orders and reports from soldiers in the field and so forth, both Union and Confederate.
Two other great resources for primary documents would be the New York Times Historical Archive site, which includes all articles, letters etc. from the Civil War era of the newspaper, and we also have online access to a popular news magazine of that time, Harpers, for the period of 1857-1865. The Library of Congress has a number of online collections related to the Civil War too, like the tintype of a young Confederate soldier shown here from the Liljenquist Collection.