James Horan has already biographed Mathew Brady; now he writes of the life and work of one of his apprentices, ""America's forgotten photographer"" Timothy O'Sullivan. O'Sullivan worked first for Brady (his Gettysburg photographs were credited to Brady in Harper's) later for Brady's partner when he separated from him. He covered the Civil War as civilian photographer with the Army of the Potomac. After the war he went West with the King Expedition, the U.S. geological exploration of the 40th parallel. He accompanied the Darien Survey Expedition into the Pahama jungles in 1870, and joined the Wheeler Expedition in the deep West between the years 1871-75. He comported himself with courage, evinced leadership ability and ingenuity throughout, according to Mr. Horan, although the Wheeler Expedition was shadowed with the suspicion of two murders. The author does not catch his man up close, but his book has value as a chapter in the history of America and of photography.