The Hidden Civil War by Wood Gray New York: The Viking Press. 314 pages. $3.75. Abraham Lincoln and the Fifth Column by George Fort Milton New York: The Vanguard Press. 368 pages. $3.50. In April 1941, when President Roosevelt called Charles Lindbergh a Copperhead, the newspapers were careful to explain who the Copperheads were. Now for the first time these Civil War fifth-columnists have been made the subject of full-length historical studies for the general reader. It is clear enough that Lincoln's Copperheads were more formidable than any that Roosevelt has yet had to face.