Smith, Whitney, The Flag Book of the United States -- The
Story of the Stars and Stripes and the Flags of the Fifty States,
rev. ed., New York, William Morrow & Co., Inc., 1975
For more in-depth material, there are also a few books on
the flags of a single state, such as Cannon, The Flags of
Tennessee [can], and there
are even books such as Sumrall, Battle Flags of Texans in
the Confederacy [smr]. There
are also articles, such as the history of the most recent
change to the flag of Nevada that was published in the 1994
copy of Raven [rav].