''Only a fool would prefer to be actively achingly dangerously unhappy, rather than bored,'' Martha Gellhorn once said, ''and I am that class of fool.'' Indeed, Gellhorn was rarely bored (she was one of the first women on the front lines of the Spanish Civil War, WWII, and Vietnam) and often unhappy (she endured a tortuous five-year marriage to Ernest Hemingway and raised a thrice-jailed adopted son). Moorehead's extensively researched biography is filled with excerpts from letters exchanged with Eleanor Roosevelt, Leonard Bernstein, H.G. Wells, and others. A perfect blend of soap opera and history lesson, ''Gellhorn'' is a vivid spotlight on one of the most undercelebrated women of the 20th century.